They might not fit the theme of "demolitions man", but the community has REALLY latched onto the TF2 mythos. To the point where it's almost like there's two demomen out there: One who blows things up, and the other who is DRUNK SCOTTISH GUY who beats you up with a bottle. This update made the second one viable! Or something.
Eyelander looks good on offense and defense because, hey, pyro is good on defense too. You lurk around a corner, and bam.
Really though, it just looks more FUN to play against an Eyelander demo, and the class is still absurdly skill-based. Against a demo noob who can't hit you with grenades/stickies anyway, you'll kill him a little easier because he starts out with less life. Against a very skilled one, he'll have tons of life and speed...but at least he can't stickybomb your sentry or air-burst you, which is a huge comfort to me.
My personal guess is that the third weapon IS a different melee/secondary, because really, the grenade launcher is a little too perfect of a weapon. I mean, if you take a demoman and get rid of his stickies, he's still got his primary 'nades and he still feels at lieast a little bit like the same class. If you take away his grenades though, it's a real big change. It could be another moderate change like the other classes get--I dunno, maybe grenades that detonate on impact with the ground instead of bouncing or have less damage more blast area--but, eh.
I'm hoping that the demoman's fourth weapon, if he gets it, turns out to be a detpack like in TFC...