What is the basis of purism in LEGO building?
I'm not too sure how to answer that. I don't want to modify any brick whatsoever myself, save for the occasional eletrical wire cut, but on the other hand I don't really care what other people do.

Considering LEGO designers aren't purist themselves (Need this part in green? Ok, I'll paint a yellow one.), why bother? Why not cut your own baseplates to the dimensions you want, when you know LEGO produces them exactly the same way, by cutting large sheets?

Then again you have different levels of purism. Part modifications. Non-LEGO usage. Non-LEGO stickers usage. Brick printing. Brick engraving. *cough*Glue*cough*. MegaBloks. And frankly, to each his or her own.

What I can say however is why I'm mostly a purist myself, if that's of any help. I don't want to modify bricks, or use anything non-LEGO, simply because I feel it would be cheating. I see building as a challenge, and each challenge has its own rules and limitations. If the part I would need to build something does not exist, then I have the additionnal challenge of finding another way to build so that I don't need it. And that's the thrill I get from LEGO building: finding the right existing part to suit my needs.

But then again, I'm not purist enough to care about what others do. Sure, I'll point a finger at them, but it's all in good fun.

Of course, glue will get you more than finger-pointing. Heretic!