A couple of reasons to look at Genesis.

There are shapes you just can't do with Mil 4 figures.

For example Anubis, the Troll, Garul, MS Lycan, etc. While you could do one of these as an individual figure it would, normally, get little to no support. When it is based on Genesis it has all the support of the base figure. 

Tri-Ax is more that just Weight Mapped Rigging. Yes it is weight mapped rigging, and yes Tri-Ax means it has different maps for each of the three bend axes. But it also moves center and end points to account for changing the length of bones. And it has a few other cool little innovative features. (Which is why Genesis won 3DWorld magazine's Software innovation of the year for 2011.)

 

Then there is the Catmull-Clark Sub-D algorithm. That isn't just make more polys, it adds polys where they are needed because of a bend and not where they aren't needed. 

 

When it comes to what Poser and DAZ Studio users in particular and CG artists in General have come to expect from human figures, Genesis is a whole new level of capability.