Going through that chart (essential is literally the only version I have never used).
Groove agent isn't worth the extra cash (IMO and a small thing...)
The stock plugins in the FULL version IMO totally worth it. Studio? Meh, I doubt it's that much better over essential. I see the difference is like 3 or 4 plugs.
I don't know what essentail comes with for VST instruments, but I almost never use stock stienberg VST instruments. I think vaccum is medocre (I hate the gui), Halion is buggy and groove agent is only good (again just IMO) for having a better click track. I have seen (and used) the extra vst instruments in cubase 5 and actually I think they are quite good.
If your using any outboard gear then the full version is DEFINITELY worth it because you can setup those busses easily. Essential, studio, LE only have options for routing inputs and outputs.
The lack of sidechain is somewhat of a drag in essential, but if you have never used it you won't miss it. It's honestly not something I find myself using in the productions I'm doing that much. That feature just depends on your need.
Extended mixer view: Yeah IMO not needed.
Channel batch export: Yeah really nice feature, speeds up workflow. Necessary? I got along with out it fine for quite a few years.
Time warp: If your doing anything with loops, your going to want this. I actually use it a lot.
In place editing (midi)/ score editor: I never use either. Some people would find both of these important. I do not.
Offline process history: Meh, IMO not that important that you only have a single undo. Most of the time if I'm doing that it's one thing. But I dunno, there may have been times where I have used the unlimited undo and not known it.
No 64 bit version for vista? I'm on mac so...