Concrete crack repair: (polyurethane) crack filler versus new concrete patch
If the cracks are due only to an initial settling, and the slab is now stable, patching with concrete definitely does have the benefit of being the same material and can be stronger.  However, this makes the whole slab one block again.
The advantage of the flexible sealants is that if the cracks are due to seasonal movement of the slab, they allow the slab to still move a bit while still being sealed (like a control joint).  If a concrete-patched slab needs to bend or adjust a bit, it will just crack itself again.