You can't use weighted pressure plates with mobs because of entity cramming
They check for too many mobs, more than entity cramming allows in one space
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In MC-108301 jeb mentioned that weighted plates should be re-evaluated, now that they have gotten basically useless or at least quite restricted in the function they had before due to his implementation of the default-on and set to a value of 24 maxEntityCramming gamerule in 1.11.

After some testing in 1.11 release, this doesn't seem to have been done yet though, thus this bugpost.

In this post on Minecraft Reddit I collected input by the community how to redesign those weighted plates so they'd be useful for the tech community again: https://www.reddit.com/r/Minecraft/comments/56y4nv/maxentitycramming_gamerule_remains_defaulton and more suggestions have been added and more might still be added, as I asked again for participation in this older Redditpost.

It'd be very kind if Mr. Bergensten could have a look into the suggestions that were and will be made in said Redditpost and maybe also give some feedback and spread the word, as his community reach is by far bigger, and people would be more likely to make suggestions if he participates in this discussion and makes people aware of it, as not everyone seems to know about it yet.

Unless, of course, he already got own plans how to re-evaluate weighted plates and doesn't need or want community input.

I might insert a redux of the suggestions in this bugpost here if I find the time.

An argument that it still detects non-living entities such as items is fair, but not satisfying, as it would deprive us from old functionality which, again, was not voiced publicly by Mr. Bergensten or Mojang generally that this would be a wanted change for 1.11.

I have not read a single word about this anywhere on Reddit (or the Mojang website/their blog in the snapshots, prerelease or release post), but in case I missed it, it'd be kind if anyone could get me a link.

The only public statement that I'm currently aware of was what Mr. Bergensten said in MC-108301, that the argument of weighted plates being rendered useless is a valid point and thus their design should be re-evaluated.

As there could be a Redstone overhaul-in-one-go planned to get rid of old bugs (not bit by bit/redstone component by redstone component as this would confuse the tech community too much and aggravate them as we could see during the 1.11 snapshots), I could live with this bugpost being resolved as "Postponed", as the weighted pressure plates would be part of a complete overhaul as well, but I definitely don't consider this bugpost a feature request or suggestion, due to the aforementioned reason.

From MC-108301:
An iron/heavyweighted pressure plate changes the signal strength it gives off depending on the number of entities on it.
Signal strength of 15 is reached with 141+ entities.

The default of 24 entities with the "maxentityCramming" gamerule renders them pretty much useless, even more so as golden/lightweighted pressure plates detect only up to 15 entities.

There was the argument that at least those pressure plates would work as intended for those people who use commands to turn off that gamerule.
As the technical community is the main target group to use these pressure plates, and as those who really can make use of those pressure plates are nearly all "decidedly Vanilla" players who would never use commands for their Vanilla worlds, it makes no sense to deprive them from making use of those pressure plates, as they are nearly the only ones using them in the first place, as highly technical players with "many entities being crammed in one spot intentionally".

I'm not a native English speaker so I cannot word it so well like Skylinerw can, whose comment over on Mojira-Reddit I will thus mostly quote, as he mentioned many things I and some of my technical-player-friends I showed it agree to:

Regardless of if the gamerule's primary objective is to boost performance or affect gameplay, it's not taking into consideration that it's blocking another feature that already existed.
Why have weighted iron plates in the game if, by default, 12 of its 15 states cannot be used without cheating?

Not to mention that gold plates already encompass some of the remaining available states for iron, even further reducing the usefulness.

A singleplayer-playing, cheats-off player taking notice of the useless weighted plate feature has no way of fixing it without using a third-party tool, which does not mean that that player knows that such tools exist and how to use them. Nor does that necessarily mean they are even aware that there is a way of fixing it.

Driving the player to using a third-party tool to fix a feature (that was previously fully available and could otherwise have been defaulted to fully available) is not something that should happen.
That also does not mean that players should "learn" to enable cheats on world creation.
Enabling cheats is optional, not a requirement for being able to fully access all survival features.
Weighted pressure plates were not cheating, after all.

The primary issue I have with it is that a cheat, by default and with no recourse for some, blocks a survival feature that was previously accessible, and requires cheating or (for some) requires third-party tools to access it again.

Keep in mind that weighted plates are not restricted to the technical community.
Everybody has access to this feature, not just a single sub-community, so it does impact everybody (including non-technical players following a tutorial that makes use of weighted iron plates).

An existing and intended feature (weighted iron pressure plates) is being hindered by default for everybody and requires cheats to access them fully again.

Possible solutions to this issue are to change the default value of the gamerule up to the highest value that iron plates can detect.
The popular solution is to change the default of the gamerule to 0 and let players/server admins decide when to use it, while iron plates become fully accessible again.

However, none of those solutions work when the gamerule is changed.

What would probably be an unpopular solution is to remove iron plates entirely, since in their current state, they are near-useless.

I'd like to add to SkylinerW's last sentence here that a removal of weighted pressure plates in favour of the "maxentityCramming" as default-on-gamerule would not only make all technical contraptions which make use of them with living entities void, but I'm sure that the reaction of the community would be even more negative than it already is towards the maxEntityCramming gamerule.