The 70-200 lens are great, but you have to have a large studio to be able to shoot full length shots of a model even at 70mm (112mm after the 1.6 crop factor). I use the Sigma 70-200 2.8 for head and shoulders shots and head shots, but usually go to a Sigma 18-125 for full length. Given a large enough studio I'd shoot everything with the 70-200 since its so sharp and gives a good comfort distance as well as great compression.
Shot of model Emilia with the Sigma 70-200 2.8. I've then selectively softened it.