I think we can look at this logically and see where everyone's mistrust lies.
1: It all starts with the owner of the game property. This is a company that will probably be willing to just let its old game sit in the vault unsold and unused rather than give it away for free.
Exception: some games are released as a PR ploy, whether to improve company image in general or to promote a sequel.
So the original game company MUST be gaining something from this.
2: Gametap is a company. It's probably not directly owned by a game publisher since there's such a great variety of games on there. Gametap has to make money, since it's not going to do this for free and it has to pay its employees and/or investors.
Gametap MUST be gaining something from this.
3: As the player, you sign up and watch a short ad every time you play. There seems to be no additional cost for you playing. Of course, you have to have an open Internet conenction, presumably so they can change the opening ad when they sell the space to someone else.
You are not spending money on this. But Gametap does get to sell ad space on your game experience, which it passes along to the IP holder.
The question becomes, is the ad revenue enough to cover Gametap's costs? You're downloading a large game, plus they have to maintain the system that updates and serves you the ads.
Something tells me this ad revenue alone is not enough to cover all of Gametap's costs, plus a reasonable profit, plus the significant amount of money the IP holder will want to serve the game.
It's entirely possible that Gametap expects to give away a lot of copies of a few good games, to get you to pay for the service and download a ton of other games. But we all know how free stuff on the Internet works: people will take what you give for free and leave, expecting to find all their other needs fulfilled for free elsewhere.
My prediction is this: A. Gametap is getting something else out of you. Is it data mining? Or B. Gametap is on an experiment which will backfire once they realize how many people take the free stuff compared to how many are signing up.
Just my 2 copper pieces.
(BTW I loved DX but DX2 sucked so hard it hurt after I finished it. Many reasons. Too painful.)