A lot of us are complaining about the Skeletal Mage's short duration. Blizzard's design philosophy here was to make the minion build more active - but here's where they went wrong.
It boils down to the psychological principles of loss aversion (ie. people hate losing things far more than they like gaining things) and framing (ie people can either like or dislike something depending on how it is presented).
Damage can be framed in different ways: a direct attack spell, a damage-over-time spell, and a Skeletal Mage are all just forms of damage packaged to appear and feel different. In all three cases, you have to keep hitting a button in order to keep dealing damage - so why is it that it’s fun to keep hitting that button in some cases, but not in others? That’s where loss aversion comes in.
In the case of a direct damage spell, the default state is that you are dealing no damage. You hit the button in order to deal damage, which results in fun. In the case of the skeletal mage, particularly in the end-game minion build, the default state is that you have all your mages out. You are striving for 100% uptime on your army of mages. So when you are pushing that button, it’s not to gainsomething - you’re hitting this button every few seconds to avoid losing your mages. And that’s not fun.
Now it wouldn’t be as big a deal to have to hit this button every thirty seconds or so, but every few seconds turns it into busywork - yes, the playstyle is active, but not in a meaningful or interesting way. "Active" doesn't trump "passive" just by virtue of being active, but by being active in a meaningful way, as with Command Skeletons and Golem.
I explore this in-depth in my full review of the DLC.