IIRC, at some point it was reported that Sweeney had the highest or nearly highest bat speed on the team. That's probably not a good thing in his case. There is a trade-off between bat speed and control and I think you can be too far on either side. Baddoo is another guy with a wicked fast bat but he can't hit enough of anything either.
But he's got a decent profile for a bench guy - occasional power a pitcher has to respect, doesn't make a lot of mistakes, runs well, is LH. He is 25 now so coming into his peak batting years. He doesn't have to get a lot better to be able to eke out a 3 or 4 year career as a backup SS.
I'm sure the Tigers wish they had a better #2 SS, but right now they don't.