Honestly, I think it should be left as it is. It would cause more confusions if you forgot what mods you set to keep on last, and entered a map with surprises. I also don't think most people turn on and off their client often enough to find it a hassle anyway.
I mean, it's just an option request, so not saying it's a "horrible" idea, but I'll have to agree with the others on it being unnecessary.