A short board without a center fin or centerboard is useless in light winds. They are ONLY created for winds strong enough for planing (80 kg, 8 m2 sail, 14 knots with moderate skills).
A short board with a center fin or a centerboard does make sense for learning in light winds, but they do not offer the cruising (gliding) sensation a longer (like 320+ and often narrower) board provides in light winds.
When it is windy there is hardly any speed difference between modern planing longboard (Phantom, Kona) and a freeride shortboard. The longboard will feel heavier and more stable in rough sees, but is perfectly fine to learn shortboard techniques like planing jibes.
On the other hand, peer pressure to be on a shortboard tends to be quite strong :-)