I ride a mtb only, urban riding I do a lot of hopping and balance work.
I 'practice' for offroading.

I practice highspeed cornering and hill sprinting on the road, this makes me a stronger mtb'er.

I do like road bikes, but prefer to ride a mtb in the city because I'm able to do stunts for fun and use the bike\skills to avoid bad drivers.
Bunnyhopping 20+ into a planted median -I like my mtb with the fattie front tire.
I've pulled some pretty ill moves to avoid cars, probably only really stuff I could do on a mtb frame\wheels.
As far as the skills crossing -totally, offroad you have unlimited learning curve for balance work.

I'm quite interested in buying\building a cyclocross bike. Seems a good choice for my riding terrain\styles, and a little more towards a road geometry.

I need more bikes.