Just one man's opinion, but with PVP, Farming, and High lvl Uber/Key runs it is clear I need to use different gear for different purposes. Storing this gear takes up space in the stash and can make farming harder because when running I like to store my unidentified items to deal with after the run. As I increase in NV with more Magic find I am finding more rares and leg items. Without space to store temporarily farming becomes less efficient. I would like to have the ability to buy more space in my stash.
You can make a lot of mule alts, they have their whole inventory that can store items...
I think it's obvious there is space for two more tabs on you chest. Thats what expansions are for.;)
I have one mule, but they are very inconvenient. I have to keep changing characters to get the gear I need. I don't believe adding more stash room would be that big a change. I have to think adding mules would be more burdensome to the Blizzard servers than just adding more stash. I would think people going in an out of games and change characters would cause some additional server load.
I like this solution: Give each char a stash of its own with only 1 tab. That way you can have a shared stash for mats, gems, organs, shared gear, general AH use, etc. But each char could throw his or her PvP gear, tank gear, DPS gear, uber gear and MF/GF gear into its personal stash. My barb doesn't need to have access to my trium and my wiz doesn't need to be able to get at the dozen or so 1h mighty weapons I wanna try on the barb.