There are several reasons.

1. I like the way V4 looks.
2. I have invested a lot of money in clothing, characters, hair, etc. and can make her look like just about any era/style character I want.
3. I love Poser and have tried DAZ Studio several times over the years and don't like it.
4. DSON did not work for me. I still couldn't really get V5 to work in Poser and I'm NOT switching to DAZ.
5. As someone who buys just about every new character that comes along, I have enough other women (Dawn, all the native Poser women since Poser 4 Pro, Michelle, Miki 4, TY2, etc.) that I can always switch characters if I can't get a V4 for the part or put them in a scene with V4 to get more versatility.

I must admit that I've been playing with Dawn, Dusk, and Luna the most lately but that may be more because the "new" hasn't worn off yet. Also, Dusk already seems to have more "normal" clothing than M4. I've been known to still use Posette on occasion. Aiko 3 was one of the prettiest characters so far and I think David was a great male figure. Why do so many people think we should just abandon the old figures for the new latest flavor of the month? I have seen some content for the newest Victoria versions that are really pretty and I'd love to have them--if they had V4 versions. I haven't seen a figure yet that makes me willing to use DAZ Studio, though. I keep hoping that one day Poser will ship with some decent looking human characters with good morphing options.