If you recall, the Rangers in Grenada (or was it Panama? I don't recall aright) had to use their telephone credit cards to call back through Ft. Bragg to get Navy gunfire support, because their radios didn't talk to each other. The USMC was using PRC-77s *long* after the Army got SINCGARS, and the two won't talk to each other if the SINCGARS is transmitting in the red.
Yes, and quite often you have different elements of the same service with differing equipment as well. Simply combining everyone won't solve that problem, good Joint headwork helps though.