I didn't appreciate Bat Form at first. I saw the speed boost and thought "Oh, I can maybe use this to get away from normal speed creatures." But it's a really amazing way to explore! From level 3 a Necromancer can scoot around picking up treasures, finding stairs, and picking his fights carefully. There's not much XP to be gained in this, but you need equipment/money/dive or climb more than XP at times.
From shape.txt:
name:bat
combat:0:-10:10
obj-flags:FEATHER | SEE_INVIS | PROT_BLIND
values:SPEED[3] | STEALTH[3] | INFRA[5]
effect
AMAGE
dice:5
effect-msg:taking bat form
blow:bite
blow:scratch
Not sure what that combat info means exactly. It's supposed to be +0 to hit, -10 to dam and +10 to AC but my to dam seems to drop much more than this. Anyway, the speed, stealth and infra benefits are obvious.