Why not replace pentalobe screws?

I have fixed a few phones now thanks to this awesome site and this wonderful forum. However I still am not sure why anyone would want to put their pentalobe screws back in their iPhone. I replaced mine with the #00 Phillips screws found in the liberation kit and I would like to do that with all my phones. However, there are so many people selling lots of pentalobe screws and talking about putting them back in after fixing their phone that I feel like I must be missing something?

Why use screws that seem only designed to keep you out and easily strip due to their poor quality when you can replace them with Phillips?

Is it an Apple warranty issue? Surely Apple would know someone tampered with your phone if they open it up even if it looks untouched from the outside...

Any real insight would be great. Thanks!