That's definitely an issue. It's mildly mitigated by the fact that you're limited to 1 positive level per con bonus, but you still get free spells out of it. Limited by your number of your highest level spell slots, and your access to restoration, which is mentioned in the energy charge rules, so works as well on positive levels as it does on negative ones. So it costs 100gp per 'fill-up' and cast-out sequence, and uses a 4th level slot for the "energize" to start each fill-up with. For an archivist (access to enervate through hunger domain scrolls...) this happens at 7th level.
Using enervation as the model, they'd last 1hr/CL, max 15. Presuming Con 24 at 7th level, you'd get 7 extra spellslots, minus 1 per casting of "energize" you had to use, and minus 1 more for the free restoration you're gonna remove the spent slots with. But restoration dispels all negative levels, so unless you wait for a few spent slots to go away, you'll have to have a casting of "energize" for each set of slots. 1d4 positive levels averages 2.5 pos levels per casting. You'd a net bonus of 1.5 per casting, but you'd have filled yourself at an average of 3 castings, so have about 4 extra slots that weren't used to "energize", and don't contain restoration.
Even better, by abusing the Sanctum Spell feat and crafting rules guidelines, you could get 2 gloves, one of 'out-of-sanctum' 3rd level command-word activated "energize", and one of 'out of sanctum' 2nd level (Healer 3 spell) command-word activated restoration. They'd cost CL 5×3×1800 for the "energize", and 3×2×1800 for the restoration, adding 5k for the expensive component. So that's 27k for the "energize" glove, and 15,800 for the restoration glove. Now, you have up to your Con bonus in slots of your highest level, freely refreshable. For 42,800gp; a pittance.
Remember that you also have ECL equal to the number of positive levels you're carrying. If you used this, the DM would definitely be awarding less XP to you.
So again, casters get ginormous benefits from something that would otherwise be fairly balanced.