I don’t recall there being any pressure sensitivity increases going from I2 to I3 (there are in intuos 4 though, so if this is important, you should save for a new tablet).
The major differences going from I2 to I4 are a slimmer, lighter pen design, some new felt pen nibs (which I highly recommend…I suppose they could probably fit in a I2 pen just as easily) an improved, more durable overlay, and the various buttons and strips on the casing from which, your mileage may vary depending on your workstyle.
If your tablet stays constantly rooted off to the side of your keyboard, and you only ever spare a single hand for it, they would probably only be of limited value.
If you’re like me though, and like to prop up your tablet easel fashion, and only go to your Keyboard as a last resort, the programmable buttons and strips become invaluable.
If you’re not going to invest in a new Intuos 4 though, I would say yeah, going from 2 to 3 would be of limited value, and you should save for a new tablet.