Linear ("additive") changes in MIDI pitch (note numbers) result in
exponential ("multiplicative") changes in frequency (Hz). Adding 12 to
a note multiplies the frequency by 2.
If you have a straight line in (note, time) and do .midicps on all the
points, you get a curve in (freq, time), but if you do .midicps on just
the end points first, you get a straight line in (freq, time).
If you're an algebraist, (note, +) is isomorphic to (freq, *):
http://en.wikipedia.org/wiki/Isomorphism#An_operation-preserving_isomorphism