They were added for atmosphere, which this game desperately needs. And to go along with the halloween theme on 1.4. I'd don't even see how they can be a nuisance like that anway. At most, they're annoying for the sounds they make.
That sounds a lot more reasonable than the person who left the thread that I was replying to, but it's a good starting point for conversation, so prepare to receive a counter-argument!
All of the things you listed are proof-of-concept additions that were never before part of Minecraft. I'd say that some of them could have been "test runs" on new features they were coding into the engine, just to ensure that they worked.
For example, with the addition of Witches came code for localizing a mob to spawn inside of a very specific space (the witch hut) which very well could have come about because of bug-squashing or expanding the future possibilities of the Mod API. This means that mod makers can select mobs to spawn only in their mod-specific dungeon, for example, without the use of proximity-based and destructible mob spawners. Not only that, but they also mean testing and bug-squashing in the realm of splash-potion-based combat. (And you can't knock the Witch, it's the most annoying/deadly mob in Sethbling's craft bros PvP map)
With the addition of the Wither came the possibility of mob destroying explosion-proof or otherwise indestructible blocks. This could be something they felt was necessary for the Mod API, too, and the only way to properly bug-remove that kind of feature is to make it into part of the live game. Not to mention that with the Wither comes Beacons, localized application of potion effects from map features, also previously impossible and likely something they added as a direct result of determining what kind of functionality mod makers will need but obviously can't utilize unless it's implemented in-game (and this is also one of those GREAT additions for mapmakers- look at FVDisco's Sonic the Hedgehog map).
Fireworks give a great framework and (semi-)practical application of creating and using custom particle effects... But they're also very festive and once again something Mapmakers have used to their advantage (see: Evil Santa boss fight).
Bats... beats the crap out of me. I guess they're just atmosphere. But they're atmosphere I surely wouldn't opt to remove. They just feel like part of the game now.
An argument can be made that any updates - even updates you like - "aren't useful or wanted", as you put it. But perhaps, and I'm not saying it's the case, but perhaps these updates came around for deeper reasons we don't fully understand or aren't publicized.
Did you ever consider that most of the updates in 1.4 and 1.5 could be abstracted far enough to argue that they are simply implementations of new engine functionality they're developing to make the Mod API and coming future updates the best they can be?
When people talk about making the "wrong" kind of updates, do they ever consider that it is not the content of the update, but the code behind it, that is important to Mojang? I certainly never considered looking at it that way until I realized (while writing this post) that there HAD to be a reason for adding "witches and bats" - I would bet real money that it wasn't just a whimsical "Hey, let's add witches and witch huts!" There was an entirely new engine functionality added, and the point of an API is that it gives access to engine functionality without letting you write your own to avoid conflicts. Isn't it conceivable that witches were thrown in as an update just so that the "public" could test out the new functionality and work out bugs?
This is a long, involved process. That's all I'm saying. They set out to write the Mod API and realized that they could not possibly make it robust enough to support the mods that are out there without creating loads of new methods for achieving things like what the Witch or Fireworks added into the core engine functionality.
these are the reasons they added all of these "useless" things I personaly love bats. they add lots of cool factor to my caves in my worlds. The sound that they make when they die is awesome too Please dont post things like this its really just your opinon not fact.
I enjoy the atmosphere that bats give but they are annoying. For players with slow computers (such as me) they take up unnecessary memory. I do not support this but we should at least have the option to turn them off.
I don't notice them at all their ignorable for me i have to say there's a lot of people who want bats gone and i just don't see the point i agree with reks227 they should be toggled like every other mob should but if it's that bad use a mod they should have a few out there...
I don't think they should get rid of bats, just give them a bit more of a practical use, like when you kill one you get an item, or xp. But they are atmosphere mobs but they do occasionally get in the way, but not often. The only problem I've had is when a bat flew right in front of an arrow I shot when in PvP on a server.