There’s the inherit advantage of having a Setup mode for rigging. Yes, I know, skelegons… bleh. Easy to use, simple Expressions with a good UI. In fact, a lot of messiah’s advantage comes from having a smart, well designed UI. (I’ve been a LW user since 5.6)
Muscle bones are a big bonus, along with bones on editable spline paths, Melt, Meta Effectors, and the whole Setup/Effects thing is accessible and extensible without the opacity of something like Maya’s system, or XSI, where any parameter can be linked to any other parameter, but (IMO) hides too much from the rigger. Everything in messiah is immediately visible while rigging, yet hideable for animating. Partially this may be personal bias: for me, messiah’s UI is like a direct line into my brain.
I still use LightWave when I just need to get something done, because I know all the keyboard shortcuts (and because of BNR), but LightWave was the reason I bought PuppetMaster 15 years ago. It has a LOT of awesome plugins, especially the Worley ones, but animating in LW, do not like.
On top of that messiah has stable, FAST radiosity. Read the 9.6 release notes. Animated radiosity in LW 9.6 is still pretty iffy.