Are you aware that the default desktop already comes with a clipboard manager ? Admittely not quite as fancy as what you describe here.
It needs to "just work!"
The goal is to address what I believe is a major shortcoming in the current default, and to improve the user experience – especially for those coming from a Windows background.
As it was described to me
There's a clipboard saving protocol already there and support in gnome-settings-daemon. If parcellite "works" when that doesn't (like for firefox) than that indicates app bugs. If you save clipboard contents without using the protocol, then you are likely "stealing" the clipboard away from apps and that will cause massive problems with something like the GIMP that exports data in many formats - either you have to pick one format (causing a loss of flexibiliy), or you have to copy all the formats (massive memory and efficiency problem).
This would make this sound like a bad idea.
Don't see a problem
I just tested GIMP with and without the daemon running. GIMP behaved the same way, regardless of the daemon, whether the paste action was performed while GIMP remained running or whether the paste action was performed after reopening GIMP. I also watched the memory usage via System Monitor and it stayed stable under 1 MiB.