There are a few things to consider. 

1.  The morphs give you some control over your final outcome, which is nice if you are fiddler.  Mind you there is nothing stopping anyone with poser pro from fiddling with the Weightmaps either.

2. V4 WM is all about making v4 into a good basic figure. After the conversion she is a much cleaner and leaner figure (No JCMs or magnets etc)  It moves really well and most of the bends are much improved over basic v4 - phantom beat a lot of the ugly out of her!  

That being said there are some areas that some find unsatisfactory.  I think a lot of people have said that the elbows could benefit from a morph to give them a more natural look when bent.  Unfortunately none of those folks has seen fit to contribute such a morph.  

3. Weightmapping is going to primarily give you smoother bends, but its not really a sculpting tool so the morphs are able to add more detail and sculpting in certain areas.

4. V4WM does require clothing to undergo a simple conversion utility (Poser Place's Outfitter).  I am unsure if any of the fix morphs adjust the mesh far enough to cause clothing to need to be adjusted.

5. V4WM is free!!  I'm not sure what your final Bill would be for all the fix morphs on the market, nor how many you would need to be satisfied.

 

Thats not exhaustive and I'll admit to not being terribly familiar with the ins and outs of the morph sets so please nobody take offense If I'm incorrect on anything.

Hope it helps