3.5" upgrade is for peeps that want real hardware, plan on playing it alot, and give zero f*cks about resale value and keeping it looking original.
2.5" replacement is for the same types of people except they'd like to keep it looking stock (at least when it's off).
Doing nothing is for collectards (like me) that would rather keep it all original and ain't gonna be gaming on it much anyway.
An emulator on your phone or cfw psp is for people that just want to game on the go with better image quality, cd support, and for minimal cost.
Keeping it stock is not just for collectards—I enjoy playing all my consoles and handhelds "stock"...it helps me appreciate what the actual hardware was capable of and what the experience was like.
Now, I understand *why* folks upgrade screens and make other modifications...and I will eventually mod a spare TurboExpress for fun.
But, I can't explain how much more fun it is for me to *know* that *this* is what the screen really looked like (I love comparing different platforms and comparing the different experiences).
I think that other handhelds (not the Express), that lacked back/side-lighting, for example, would be prioritized on my list of "desirable mods" since natural/artificial lighting doesn't always allow for an ideal situation. I think Gameboy Advance is really, really lame. I think Nintendo was LAME for not shipping the original GBA with side/back-lighting.
That said, I still enjoy playing my original GB for many, many games...so, I really think it depends on how much of the game I feel I am "missing" or how "needlessly frustrating" certain parts of a game are (platforming, for example)...
The only mod that I have no problem with would be BATTERIES! I don't think that modifying a handheld for better/more convenient battery life betrays my desire for "playing" the games "stock"...