I can't even begin to list all the games I've bought on sale that have become some of my favorites of all time. It's a lot.
The main problem however is this one: by buying any game you cast a vote. You give money to the developers or their publisher and this way they know which projects are good, which one show the right direction for the future. If you just pay for games that you don't even bother to play, this evolutionary process is being disturbed.
For example this might happen: some indie developer made some game, hundreds of thousands of people buy it just because it's on sale. Seeing the success the developers assume that their game was awesome and people want to see more like it. Then they develop a sequel or something but it fails epically. People don't really bother buying it. They actually bought the previous game but didn't like it at all or aren't ready for a sequel yet because they don't know if the previous game was any good as they had no chance playing it because their games library ist just too huge. As a result we either get worse games or even destroy some developers just by being ignorant consumers. That's terrible!
But sill my main problem is that I generally loose respect for games, even the good ones, when I'm drowning in them. They really do loose value.