The Odinson might be the God of Thunder, but he's not the only one who's wielded the hammer Mjolnir in Marvel's history.
Yes, while Thor's hammer might be just that - Thor's hammer - more characters than he'd care to admit have taken control of the object and the power it possesses. It's a trope comic book writers have utilised repeatedly over the years, and while some characters are infinitely more worthy than others, one can't help but feel that maybe - just maybe - some of them shouldn't have been up to the task.
Still, there's no denying the value of seeing someone other than the Odinson wielding his weapon. It's a massive feat to accomplish, and there's a reason why writers keep returning to that concept. People look great swinging around an ancient nordic weapon, and when it's a hero - or villain - you don't expect, you begin to understand the gravity of their accomplishment.
It might get a little less impressive each time it happens, but when the hammer looks as good as it does, can you really blame writers for swinging it around so often?