Not to mention the massive improvements for devs. You no longer need patches in individual installers. That hotfix ready? Bam, in the game. You don't need to wait until the other five problems are fixed that would warrant a proper patch.
You apparently don't know how software development actually works, that couldn't be more wrong.
Doubt it has much to do with that. The clients offer many needed features for modern consumers that GOG cannot emulate.
That's the main reason why I am convinced that GoG really should develop and offer an optional client.
Heck if Microsoft had a clue they should have created long ago a digital distributor agnostic game launcher/updater/with achievement/etc... either integrated in Windows (or downloadable) with freely available APIs allowing all game publisher to integrate in it... but the only thing they managed to come up with was the mostly broken and closed GFWL and the ridiculous windows 7 game launcher...