Why wouldn't you? It helps them to be inspired to make more of an effort to listen to the fans.
You no longer have to live in shame knowing you've got a pirated copy.
You will legally own it.
It also makes a neat collection item for nostalgia's sake.
You can legally sell a 2k3 game.
You will gain legal usage of the RTP that came with it so that if you want to use it in other RM engines, you can.
You'll be supporting the people who support your hobby - it's almost like you'll be supporting your hobby yourself.
You will no longer be one of the dregs of society who pulls down the rest of your hobby-kin.

Are those enough reasons? Or would you like more?