The most USELESS monster.

What's the point of mobility if Holy Call and Plague take them down in one or two turns?

Mobility also means movement on the map. Skeletons - 19, Imps - 28 (almost 50% increase to the skeletons' movement, which isn't that little) :D.
If you put Imps with a hero who has no movement penalties in his army, he will move not 22 steps, but 28 (6 steps further, meaning he can cover more territory) :D.
Spending the enemy mage's turn on the Imps (if he wants to), is also a good strategy :D.
Someone here correctly pointed out that you can also steal a bit of magic from the Angel or Phoenix, so they don't have enough to resurrect, which is also good and you don't have to spend a turn summoning (imagine 30 angels without resurrection, which are overwhelmed by 50 Imps or 5) :D.
P.S. Skeletons also have their advantages :D.