I sort of feel that the question of detentions being 'too light' is not addressing what was actually going on. There was a lot of other stuff going on in Harry's world at that time, and it wasn't just detentions, and not being able to play Quidditch, and not being able to be with Ginny Weasley, and not just being away from what he was wanting to do, and not just having to be in the company of Severus Snape, and not just having his nose rubbed in the actuality that he had acted as a liar and a cheat --- having the detentions, repeatedly, I think drove in the point that Harry blew it trying out a spell which he didn't have any idea of what it was or what it did.
I don't remember any other time before this, that Harry was terrified of what he'd done, terrified of the outcome of what he'd done. "No---I didn't---" and "Harry was still watching, horrified by what he had done, barely aware that he too was soaked in blood and water". I feel that it was very possible, if not probably, that all of these weekends were opportunities for Harry to start waking up to his responsibilities as a wizard. Maybe this was not at an intellect conscious level, but underneath it all, he KNEW why he was there and that all of the outward, ostensible punishments given by Snape had nothing on what he was, internally, having to deal with. I feel that Snape (and Dumbledore) knew this, and they were aware that the punishment was more than just what would be apparent to others.
As for Saturdays ... well, that is Quidditch games.