Pretty sure you can't get a good number of mutas out in time, especially on one base.
Also I seem to be missing the point of runbys, if they push can't they just win the base trade?
Dam I need to play more
I think mutas are better on 1 base, since you usually have the element of surprise and can do massive early damage, and will obviously hold back a mass rauder rush. He needs a good amount of rines to stop mutas at that time, and if he gets a split MM army u can opt for banes sooner instead.
Runbys dont have to be all-in. You can send half your lings to hide somewhere and have half at home for a decent amount of defense. Lets say we're playing xelnaga caverns where im at the top and the opponent is at the bottom, and Im holding the bottom watchtower with a ling.
If I see his army moving towards the center I can either 1. Bring the lings back if I DESPERATELY need the extra defense (which isnt commonly the issue), or 2. slip lings behind/around into the main/nat. Now he has to either hope he has enough to hold it off (not likely if its early game), or turn his army around and stop pushing, or attempt a base trade. Either way theres a huge chance hell lose plenty of workers, and since your the one scouting earlier and being aggressive earlier, your in the lead, because as im sure most people know its easier to defend than it is to push.
tl;dr version, what would you rather do, fight army vs army? or fight army vs army with your lings killing his workers. Since zerg have generally less options than the other 2 races they have to be able to dish out damage before they can spread and gain a nice lead.
Also this only works if you SCOUT SCOUT SCOUT. The better you want to be at this game the more effort you have to devote to scouting