the main advantage of UME is convenience if you're used to MAME, general use of MAME and where to find MAME resources etc. then you use it in practically the same way (and can consider it a version of MAME with bonus support for everything from MESS)
As far as emulation goes UME/MESS is actually perfectly acceptable for a good number of cases*, and is improving all the time, although does still leave a bit to be desired for some popular systems and some usability issues due to MAME's full-screen interface.
However if you're using something like Hyperspin, and already have scripts for other emulators then it's probably just easier for you to use those in the first place because you're not really going to be benefiting from the added convenience, or MAME philosophy of something like UME/MESS.
Maybe that makes you think the whole MESS / UME thing is a bit pointless, but let me explain..
The MESS / UME development is the future of MAME anyway, and most development on the project is heading in that direction (if you look at work done in the last week 95% of it is to benefit MESS) so you could also see it as a preview of the future. Of course you're got the guarantee that the code will always be available, and anything known will be documented in a public way, no secrets, in the consistent and fairly easy to understand MAME codebase. Likewise this means that it can be ported to any new platforms that come out (and is thus futureproof) and provides a place where people can properly document ROM dumps from cartridges and stuff, rather than the hacked up/merged rom data+header approach most console emulators use. Development of the emulation of many of these systems also leads to futher improvements in the arcade emulation indirectly, by setting a higher bar and providing more rigorous test cases for the cores shipped as part of MAME.
However, like I already said If all you want to do is play games in the short term those values probably aren't going to mean much to you at all even if it's a positive movement as far as emulation goes ;-) Of course, if you have time, and like to tinker then UME/MESS gives you a lot more to play with, and shows a lot more of the potential of the emulator we have than the arcade emulation alone, so for the technically minded, curious, historians and archivists amongst us it's a dream. (nothing quite like being able to compare 10 different ports of Pacman within a single emulator)
* For example, if you grab the CHDs then the NeoCDZ emulation is just as good as Final Burn Alpha, and only really fails on the 2-3 games it does because we attempt to emulate the CD controller rather than hack around / bypass it like most of the other emulators, and for some of the computers (eg. Apple II) it's basically the best emulator out there due to the flexibility offered, however for something like SNES while a lot of games run fine things using the SuperFX chips have significant performance issues. Recent improvements to the Playstation emulation make it very competitive too, you can run something like The Raiden Project / Raiden DX to play Raiden 2 (the arcade version in MAME still doesn't work) and make use of the rotation controls provided by the MAME engine in conjunction with the TATE rotation controls in the game to play it in the native vertical resolution, you also don't have to worry about plugins etc. this is standard functionality.