I haven't seen any discussion on this yet but I think it's something that needs to be changed. Having played several games already in which a teammate has dropped, I can say that it's quite frustrating to try and guess where his supply is at. You end up massing overlords/pylons/supply depots just to ensure they don't get supply capped. Controlling two players is hard enough; controlling two players when you can't see the supply of one is quite a bit harder.
I'm guessing Blizzard expects players to use the dropped player primarily for income (which is a valid strategy). In WarCraft III, teammates couldn't use the dropped player's resources on their own units/buildings but they could see the gold/lumber/supply and control that player. StarCraft II is different because there is no upkeep system and so Blizzard probably thinks the disadvantaged team can just use the extra income to build up armies faster.
As mentioned, while it is a valid strategy to use the dropped player exclusively for income, they should allow us the versatility to be able to play the dropped player standard if the teammates are skilled enough to do so. Being able to see the supply would help quite a bit in order to keep up with timings, etc.
PS. I wrote this only keeping in mind games where players drop at the beginning, because that's what happened in my games each time. In the late game it would be a lot more beneficial to be able to see the supply, as the main and expos should already be saturated and the focus is more about building up armies.