En route glide slope?

I was staring at the VOR-head during a flight a while back, and started
wondering why we only use half of it most of the time. Whether by GPS or
VOR, the CDI provides us with lateral navigation. But the glide slope
needle sits unused until the very end of the flight (if then {8^).

Why?

Using older technology, why not have an altitude bug and let the bug and
altimeter feed the GS needle, providing VNAV information en route?

Will WAAS-ified GPSs be able to drive the GS needle en route?

Curiously...

Andrew