First, allow me to start by saying that I understand this is intended behavior. However, the behavior is not proper considering Adventure Mode's original behavior and its intended purpose. The changes made to Adventure mode break existing maps made prior to 1.4.2. You might call this an annoyance rather than a bug - see MC-2302 for why I feel this is a valid issue to post here.

What I expected to happen was:
Players would not be able to place or remove blocks in a map if they were in Adventure mode.

What actually happened was:
Players in Adventure mode could place blocks freely, and remove them if they had the proper tools.

Steps to reproduce:
1. In Survival or Creative mode, get some blocks and their corresponding tools into your inventory.
2. Change your game mode to Adventure (/gamemode 2).
3. Observe that you can place blocks.
4. Observe that you can remove blocks, as long as you attempt to use the correct tool.

In its original inception, Adventure mode did not allow the player to place or remove any blocks whatsoever. This change contradicts Adventure mode's purpose on many maps, as "you may not place or remove any blocks" is a much more common rule in adventure maps than "you may place any block you have, and remove any block you can get the correct tool for". Many maps in 1.3.1 had this more common rule, and the changes in 1.4.2 break those maps by allowing players to cheat.

Some people may suggest the map maker must simply prevent players from acquiring tools and blocks, and the problem is solved. However, it is not simple to do this at all. Spiders, a common hostile mob, drop string, which may be used to craft wool which can then be used to cheat in parkour or escape the map's boundaries. Maps which allow the player to craft weapons must inadvertently also give them the power to create tools which can break the map. In fact, even swords can be used to break a variety of blocks. Thus, there is no way to escape having to put the "you may not place or remove any blocks" rule in such a map, and such a map is doomed to fail on larger SMP servers unless plugins which specifically prevent block removal/placement are used.

The ability to place blocks in Adventure mode also comes as an annoyance to players in some maps. I have seen a few videos where a player accidentally places a wool block they wanted to trade to a villager, and the block cannot be removed without acquiring shears or using commands.

While the new Adventure mode has its uses in maps which do not bear the "you may not place or remove any blocks" rule, the old Adventure mode supported its own massive share of maps (if anybody is willing to do a breakdown of the percentage of maps with this rule, feel free to post it in the comments). Furthermore, it supported modless multiplayer adventure maps, where a large amount of players can safely be allowed to play, whether or not they could be trusted not to cheat, because it literally prevented cheating. With these changes, such servers are broken and must get protection plugins to prevent cheating.

Thus, although I agree the new version of Adventure mode should not be removed, I earnestly feel the old version should be reinstated somehow. Whether it be as a new gamemode, or whether gamemode permissions can be made configurable, it would be a boon to map-makers and server owners alike whose maps go by one of the most common rules to be seen in Minecraft adventure maps.