Well, DEN is supposedly high on the VX list of target markets, so yeah, planes and gate space would be nice, and maybe you'd want some F9 employees as you expand, but a merger with F9 is just adding additional ways to bleed Sir Richard's venture capital at a faster rate, on top of too-expensive oil. Part of making something work with finite VC resources is being really, REALLY good at saying "no" to bad ideas (and saying "yes" to the right ideas).
I'm also not so sure they'd take FLL over MIA as their first Florida market. Doesn't VS fly to MIA? Isn't part of the idea behind VX to be able to feed into the other Virgin airlines?