Dretches getting killed on the far side of human base, and then returning before humans can cover the short distance to their base and back can be a big advantage.  They don't expect you so fast, and you can prepare an ambush (pre s2 

 )
edit: Lots of people evolve in base then go to attack the human base.  Not me, I'll strafe jump a dretch to a location near their base then evolve.  Some maps (Like UTCSB ) you can evolve around the corner of the low side.  Humans don't see a tyrant go down there, only coming out a little closer to their base.  There are lots of such locations, and as a dretch you can get there very fast.