I think that a limit would be bad, after all GW makes a point of being about player skill rather than time played, and the health loss means that you need to be skilled to keep a lot of minions up.
Besides, if they imposed the limit you suggested, which would only allow 5 minons at lvl 15 death magice, then i would lose 15-20 minions, which i think would be very detrimental to my build.
The health degen makes it about being a smart necro and knowing how to keep alot of minions alive. Also, the ten health regen is nice, but not enough by itself. You also need BOTM, or if you have monk secondary, area heal. Because at lvl 16 the regen is only 21 seconds, whereas the skill recharge is 30 seconds.
And the longer a minion is alive, the faster the health loss gets, to the point where they die in a matter of seconds.
So I like the system the way it is, it allows you to be more effective than if there was a limit imposed.
Plus it pisses off my roommates when we quest together because I go half their speed, stopping every 10 seconds to do an area heal or Veratas Sacrifice, and they dont like to go on without me cause my boys reduce thier chances of getting hit about 20 times over.
Edit: P.S. I agree that they are stupid, I can't tell you how many times I have lost my army because they stopped moving somewhere and just stood there untill the died