I had some initial trouble with MGS3 Snake Eater's controls, but I got used to them in time. I could see how the CPP would help, but with an XL, the import CPP XL/LL would cost $30 before I even got the game.
I also have Resident Evil and Kingdom Hearts, and I heard it's only good for lefties in Kid Icarus.
I can't play Resident Evil Revelations without the circle pad pro. Kingdom Hearts I dunno cause the demo doesn't support circle pad pro, MGS is fine either way. Kid Icarus it's only needed if you hold the stylus in your left hand
Don't hate me because I'm beautiful ~ f00had http://i50.tinypic.com/14ie929.jpg http://www.youtube.com/watch?v=xE5c8ugVYVc
KH3D plays like BBS on PSP without the circle pad, it's not a big deal imo, depends on personal preference.
For KI:U I'd say it isn't useful unless you're a leftie, because aiming with your left hand would leave you without a circle pad for movement (a pretty bad thing), if you aim with the right hand it's not that important, you can't aim with the extra circle pad iirc.
RE:R has a ton of control options, and I personally love the one with gyro aim, I think it's better than aiming with an analog stick, it kinda sucks if you want to have the 3D effect on though. Otherwise you can also aim using the left stick, you won't be moving when aiming unless you press another button but that's like playing RE4 or 5. The circle pad pro offers a good option, because you can control the camera and move while aiming, but it's hardly necessary, you may find a good control option easily without it.
tl:dr - not necessary, but may be better if you like having 2 analogs.
One thing to note with the CPP on KI:U, you can change the fire button to the Zl trigger, and the skill button to L. That, plus the extra grip, personally makes it ten times better to play with it, even though I'm not a lefty.
I'm not a lefty, nor do I actually own a CPP, so some of these opinions might not matter, but: KI:U- No need for it, plays fine without it. MGS- Only played the demo, but I got used to it without. RE:R- Plays OK without it, but I imagine it would be better with one. KH- Never played MH3G- Borderline unplayable without it in my opinion.