Well, personally I don't think it is entirely needed, however it is a way to make it more interesting.
I know that low Piety isn't a bad thing on it's own. But it would be fairly bad if you try to convert other religions, and it is when you expand into other religions that you are most likely to need the money (warring). But the Piety loss would mainly be there for RP reasons, it is the increasing stability costs that would be the problem. And if you default again you can run into perhaps stability loss itself (I specifically didn't specify what the direct losses could be in order to not set people's thoughts on any specific path, but with a forced hand I prefer stability), in which case you are now saddled with loss of stability and the increased stability costs. It would be more costly to not pay the loans, but it would also be less problematic to take loans (no interest or inflation). I think those things balances out nicely. And so what if that would make Muslim states a bit stronger overall, it isn't as if they don't need it, even the Ottomans seems to get their teeth kicked in regularly.
The main problem I can see is if the AI can't handle loans properly, which would result in much weaker Muslims. But it doesn't look like that happens at the moment.