I think I can answer that... Some, like me, don't want the hassle of frequency coordination and prefer the broader use allowed by GMRS. As I understand it in my AO, frequency coordination has locked up most amateur radio repeater pairs. Some are camped on the pairs without even having a repeater on the air for years/decades. When I checked into it some time back, I was told "Good luck getting a pair!" by local hamsters.
We also use GMRS for actual work; can't do that with amateur. Additionally, there is the big appeal of immediate family members being able to transmit under the one license; no tests required.
Frequency coordination is not really official. They really have no power to stop anyone from doing anything. All the FCC cares about is interference between users. FACT, if someone has a parked frequency pair, for amateur radio, but it's not used, and you put up a repeater, the FCC will not give a damn. If there is no interference, there is no problem. frequency coordinators do not own the frequencies. If they file a complaint with the FCC and there is no interference, the FCC won't care.
To the person who wondered if DMR will work on GMRS, of course it works just fine. In fact, if DMR is ever adopted, it will greatly expand the use of GMRS. But FCC is always making sure that changes are backward compatible. Fact is DMR would reduce a lot of interference. That's what it's designed to do, and you get double the capacity, if not more. Two conversations can take place simultaneously on the same frequencies.
Regarding how long it takes to transmit a text. This example is "I'll be headed home soon". It was transmitted in DMR message, into a dummy load, and i real time speed. You can hear the beep, when the message was sent.
But being legal is an entirely different matter.