If you're a great-weapon fighter you are already taking a hit to AC (no shield), and the trade-off is (in theory anyway) the additional damage you inflict.
Now, you can choose to wear medium or heavy armor (AC 17 with half-plate and DEX 14 which is reasonable, or 18 with plate). By contrast, Studded Leather with DEX 14 would be AC 14. Of course 3-4 points lower is a big deal, but again that is supposed of be the trade-off.
This sort of a rule would remove the trade-off, so you could max out AC 18 (eventually... most games don't get this far anyway).
EDIT: This
would actually be a better feature for GWF style IMO than the minor bump to damage it offers.