Things to note:

GMRS/FRS is locked to specific channels which could get crowded (although the FCC has recently expanded the band).

HAM is not legally allowed to TX over GMRS/FRS, but CAN RX.

HAM generally has more power output (30+ watts) than GMRS and can reach out better when you have clear LoS.

Repeater advantages have already been discussed, but there's also IRLP for licensed operators to use.

I personally run a Yaesu Ft-8800R rig in my 4Skinner and carry two Yaesu VX-5R HTs for in the field work between operators.

You don't need a massively complicated set up to get the benefits of HAM. For several years, I ran one of the HTs with a Diamond antenna and a hand mic. Granted, I was maxed out at 5 watts, but with the larger antenna, I could reach out beyond the 5-7 mile range.