Game clients can be very helpful. I too agree that sometimes managing downloads from different services can be a little cumbersome. Nothing a good spreadsheet and some organization can't overcome. :)
Ideally, a game client would allow both automatic install (without downloading a package installer first) and download of installers that can be backed up. This is more or less what Desura does, I think.
For the most part I'm happy with the variety I have. GOG - back up the installers and bonus content. Steam - keeps my library backed up for me. Desura offers both options. Others like Amazon and Gamersgate, where I have just a couple games each, it's mixed. But overall, not too bad. :)
GCStar is better than spreadsheet.