Seems to me this is rather the digital equivalent of Distance Selling protection for physical media, provided you don't break seals/shrinkwrap. I.e. it's not about trialling/testing the game, any more than you can if you bought a DVD in a store, unless the store chooses to allow it.
Where, I guess, it does offer some protection is for buyer's remorse over impulse purchases .... as long as it's not installed, etc.
Ultimately, though, the choice, dear reader, is yours. Steam offer a service, and either you accept it on their terms, or you don't. Nobody is compelled to use them, so here's a radical thought .... if you don't like the way they do business, if you don't like the terms under which you hold an account, don't buy from them. It ain't exactly rocket science.
Personally, as I've said many times, I don't accept the basic premise of their operating model, which is that many games require an 'account' and that access to games you've bought and paid for can be rescinded at any time and at their discretion by locking or closing that account. It's not my only objection to Steam, but it's enough in it's own right that I'm not paying them money on that basis.
So you lot have a choice. You can moan and wail over these changes, including them adding in the onerous bit on proxies, AFTER many of you have handed over large sums for games previously bought, but ultimately suck it up and hand over money anyway. Or you can refuse to buy any more. I know which way I'm betting most people will go, which is despite the moaning, you'll pay up anyway.
In which case, I don't have much sympathy.