The most recent peak of concurrent Steam users was over 6,000,000 people. According to your argument, that is 6,000,000 people that have willingly installed 'spyware' on their system, and are using Steam in the knowledge (or ignorance) that their software could be taken away from them at any minute. That would either make Steam the most ingenious piece of malware ever or, a bit more likely, a convenient and trustworthy solution that has made gaming more secure and convenient for publishers and consumers. Like it or lump it, solutions like Steam are the way forward if you want to play recent games.
As far as what details Steam collects, to my knowledge, and by the wording of the privacy agreement. it's only usage statistics and hardware information.