Pros and Cons of Halo 3 on Steam for Microsoft
Pros
Money
Get more people playing and talking about Halo
Digital distribution is much cheaper than retail
Cons
Any future HD/remastered versions are less relevant
Halo brand becomes less Xbox exclusive
Have to share money with a competitor Valve
I would probably do it if I were in charge. Halo 3 is almost 6 years old, it's not going to take much of a bite out of the next or current gen Xbox.
Pro
Giving Fans finally good Halo games back
Con
people stop playing their shitty new Halo games.