I can see where this would be handy. I did some looking into this, and saw where the problems might arise. Just to make sure I understand though - you want a function that would do something like this for you, based on a "slice direction"?
If so, how would shapes like these be sliced?
If it's only against edges that are aligned to an axis, that should be fairly straightforward, but once you throw in an odd angle, it starts to get complicated,
However, I could see some opportunities for a function that cut across selected, contiguous surfaces. In other words, say you have a cylinder where the caps are facing +y/-y, and you want to slice it in two along the X/Z axis (creating 2 short cylinders), like so:
You could go into Surface mode and select only the surfaces on the "sides" of the cylinder, and this function would create a new slice point only on edges between which both surfaces were selected. It would ignore edges in which one surface was selected and the other was not.
This approach would require you to go into Surface mode, and would require you to select the surfaces between which you wanted a slice. However, the angle of the object/edges would no longer be an issue. Plus, with the new select loop functionality coming up in version 6, it would be very trivial to perform such an operation...
I'm still winding up another project, but I may end up writing a plugin for this soon --- it's the best alternative to the Knife plugin I've seen yet --- I'd definitely get use out of it.
Dennis