I think a Steam like client for Amiga (cross platform between the different flavours) is a great idea.
I dont really see how Steam spesifically can be useful for Amiga since there aint anything available for it to purchase. Yes, its a really good chat client, I agree.
But if anything; Amistore or phoenix's store client should be adapted to have such a low impact chat client included.
EA's attempt for a store/chatclient, Origin, was aweful. Tho its over a year since Ive used it since Ive not played Battlefield or games much in general.
But Steam I pretty much load up every time I power on my Wintel PC.