Druids aren't exactly known for their swift, deadly openers. Sure rogues with ambush are an obvious concern with 10 debuffs, but with the current rules, you can have 2 rogues in a premade as long as you have no druids.
It is much harder for a druid to get a good opener from stealth than it is a rogue. I'm not saying there should be zero concern about it but it seems to me you'd still have bigger balancing problems with priests, paladins, and hunters than you would if you had 2 rogues and 2 druids.
And I'm well aware of the fact that premades generally have not allowed swiftness potions in the past. What I want to know is why it was decided to disallow them? Some classes such as warriors, paladins, and warlocks have no real mobility and swiftness potions would benefit them greatly in certain situations. Why is this not allowed? Is it simply a matter of the high number and cost of the potions? Do they really cause severe balance issues? If so, how?