Villagers who are other professions than farmers, jobless or nitwits, will pick up non-food items such as wheat seeds and beetroot seeds.
This may lead to their inventory getting cluttered rendering them unable to pick up food and therefore breeding with others.

EDIT: The non-farmer villagers will share these seeds with any farmers they meet but will keep a minimum of 24 of each in their own inventory. This is problematic since they will then never be able to get rid of these.

Interresting enough, non-farmer villagers are not able to pick up bonemeal, only farmer villagers are. The same mechanic should be implemented with other items, if the villager have no practical use for the item, it should not pick it up.

In the attached screenshot it is seen that the villagers inventory is full and it is therefore unable to pick up any of the potatoes lying by its feet.

UPDATE: I have found that if a non-farmer villager picks up 8 stacks of seed (any kind) and meets a farmer, it will share most but keep 24 seeds in each inventory slot, thus effectively prevent itself from ever picking anything else up, unless it itself becomes a farmer and then can use those seeds. (See screenshot)