A lot of skating moves are done turned in (like spirals) that ballet need turn out for. In genera, turn out in skating is BAD, and ballet dancers tend to turn out for everything (like spins, in air jump positions that can lead to wraps, etc etc...).
Also turns are done completely differently (spotting, etc...) and doing the two at the same time is very confusing.
I'm one to think serious skaters shouldn't do too much ballet. One can learn elegance and musicality without it, and there are other, more easily "combined" dance forms such as jazz or lyrical, that would be less obtrusive to skating technique than ballet.
Just my opinion. A LITTLE ballet never hurt anyone, but too much ballet isn't good for skaters.