Why isn't the clipboard a stack?

For 22 years now, graphical OSes have offered the same puny little single ref-cell as the model of the clipboard, i.e., the storage for the copy-paste mechanism. Yet most of the interaction I have with my computer is stack-shaped; start performing a task, realize it requires a sub-task, perform the sub-task, and return to the previous task. If I happen to have copied something to the clipboard which I am pasting in various places and then need to perform a sub-task in the middle, this single register may become trashed in the process, with a "user-save register policy" as my only recourse.

I know Microsoft Office has some sort of fixed-width-but-bigger-than-1 array for its clipboard, but it's only within Office, not OS-wide.

I'm not saying the design of a stack-based clipboard would be obvious--for example, I don't know how the system should know when to pop an item off the stack--but a single ref cell isn't enough.

Work in progress

Three productivity boosters

A few of my current favorite productivity boosters:

1. Subversion - With CVS, I wasted a lot of time fretting about file names and directory structures, because it was such a pain having to muck with the repository when I changed my mind. I ended up not being able to prototype nearly as rapidly because I spent time trying to plan the overall structure of my code. Sometimes I'd even give up and go off to work on something else. With Subversion I just blithely give things whatever name and directory structure suits me at the moment and keep working.
2. Voo2do - This is the best task-list and project management software I've found online so far. It's got the right features and no more: I can give tasks a name, an associated project, and optionally a priority, a deadline, and notes; then I can create custom views over a particular set of projects. And that's about it. Now I'm keeping all of my to-do lists for everything in one place. Since I've been in grad school, I've found I do a lot of task-switching between my various jobs, so being able to keep all my tasks listed in one place should help me maintain my priorities not just in the context of one job but amongst all of them.
3. Procmail - You've just gotta keep separate folders for emails from separate projects.

The name of the game appears to be unobtrusiveness of the technology (K.I.S.S.), a high payoff-to-learning-curve ratio, and the ability to reorganize continuously.

I'm still on the lookout for good calendar software. Ideally online and compatible with Apple iCal.