Actually, it turns out taking down one tower too early can actually be detrimental. Without that tower, your lane tends to push way too far past the river, meaning that you can no longer stay in lane safely to last-hit and get XP.
This forces you to roam in cases where you might not be a strong roamer, as many champions aren't at low levels.
I would also say that duo mid is less viable than duo top because it's easier for the enemy team to defend mid, and harder for a duo team to zone out and deny the single laner because of the smaller lane. Unless you get the tower by 6, you've usually wasted a lot for very little return.
If you're planning on just farming mid, you should do solo mid duo bot - the point of duo mid is to push that tower for map control and it doesn't help if your team isn't going to capitalize on it.