Parents can't win, if they buy a present that's useless - 'yet another' naff teacher gift/chocolates/box of homemade biscuits - then they're in the wrong, if they have a collection for vouchers then they're also in the wrong!
I've usually got something individual for dc teachers, but if someone offers to collect for vouchers then I sometimes do that too.
I think joining together can be quite a nice thing to do, as the teacher ends up with something that's more of an actual treat for them - they can choose something they really like that costs a bit more. It's not just 'can't be arsed bung some money in an envelope'! My kids also always do the thank you cards anyway, even if we're also contributing to a collection.
For the last year the teachers and TAs have been key adults in my kids lives and the thank you gift is not just about 'have an extra gift for doing your job' but rather a kind of goodbye as well, acknowledging the significance of that role in their lives.
It's no different to giving a box of chocs to the nurses on a ward after a stay in hospital - the nurses have just done their job, they're not saints, but they have still been very significant for a short time and people tend to acknowledge that.
I think it's a bit different when each teacher individually has a more transient role, like at secondary, but primary isn't like that.