I like this. Although it's got easy exploits - something as simple as a Restore Magicka potion could potentially be your ticket to a legion of summoned beasties.
I would propose a slight tweak. Instead of constantly damaging your magicka, make your conjured creature drain your magicka by a set amount. Your conjured creature is permanent, until it dies or you dispel the conjuration. But your magicka pool is now lower (A constant drain - not damage - to again reflect the concentration needed), so you cannot cast as many other spells. And you can conjure additional creatures, too, with further penalties to your magicka pool. And so, if it cost 20 magicka to summon a scamp, and you have 120 magicka available, you can have a half-dozen scamps scampering about you - permanently. But if that's the case, you'd have tapped your entire magicka pool and would be unable to cast so much as a flicker of fire.
This eliminates the Restore Magicka exploit, though it does have a Fortify Magicka exploit in much the same vein, but in my eyes to a lesser degree. That is potentially mitigated by making "fortify" potion effects rarer in alchemy, or by adjusting the magnitude of the potions to be less than it was in Oblivion.
(Alternatively, you could keep this system and apply penalties for each conjured creature beyond the first, so that (using the scamp example above) your second scamp costs 25, your third costs 30, your fourth costs 35... Meaning you'd only be able to have a 3-scamp entourage.)
I would propose a slight tweak. Instead of constantly damaging your magicka, make your conjured creature drain your magicka by a set amount. Your conjured creature is permanent, until it dies or you dispel the conjuration. But your magicka pool is now lower (A constant drain - not damage - to again reflect the concentration needed), so you cannot cast as many other spells. And you can conjure additional creatures, too, with further penalties to your magicka pool. And so, if it cost 20 magicka to summon a scamp, and you have 120 magicka available, you can have a half-dozen scamps scampering about you - permanently. But if that's the case, you'd have tapped your entire magicka pool and would be unable to cast so much as a flicker of fire.
This eliminates the Restore Magicka exploit, though it does have a Fortify Magicka exploit in much the same vein, but in my eyes to a lesser degree. That is potentially mitigated by making "fortify" potion effects rarer in alchemy, or by adjusting the magnitude of the potions to be less than it was in Oblivion.
(Alternatively, you could keep this system and apply penalties for each conjured creature beyond the first, so that (using the scamp example above) your second scamp costs 25, your third costs 30, your fourth costs 35... Meaning you'd only be able to have a 3-scamp entourage.)
An exploit will always be uncovered, though I agree that your method is less likely to be exploited by the average user. I'll be happy if either this or the having the summons turn on you ideas get implemented somewhat.