If you can get - for the same launch cost - not 70 tons, but 1400 tons to orbit, even if they are in 54 ton, not 70 ton lumps - it starts being really questionable what the benefit of the 'shuttle derived' heritage is buying you.
It stops being questionable when you stop comparing prices - and start comparing what you get for those prices. Splitting the payload increases the amount of parasitic mass, I.E. non payload mass such as the support systems the payload requires until joined up in the final assembly. Splitting the payload also considerably increases the total risk of the mission - both by increasing the number of launches required and by adding rendezvous, docking, and assembly steps not required by a unitary payload.
Seriously, while I can't and won't argue the SLS is a good idea - there's a lot more to the equation than simply price. My minivan is much cheaper than a full sized pickup truck, but nobody sober and in full possession of his senses would ever confuse one for the other. No sane person would ever send one to do the other's job.