I think the crack itself is not the problem, but perhaps the reason it was cracked before. Something caused it to crack. An overzealous mechanic can crack one of those pistons if it's stiff and doesn't want to retract well, and it's possible that some other brake problem occurred. It might be a good idea to go back in there, make sure the piston moves freely, and also make sure that the rails on which the pads slide are reasonably smooth and free. They can pit, and cause a pad to hang up and not retract. The pads should be a firm fit on the rails, but they need to be able to move too. Of course when you open it up you should also just double check to make sure you put it all together right, and that there isn't something visibly wrong with a pad, and that there isn't a kink or bend in the dust shield that is scraping somewhere, or a little bit of dislodged rust interfering somewhere. It doesn't take much to make a perfectly good brake noisy.
If the crack is too large, it might influence the way the springs on the piston-side pad fit into the hole. Check how everything is fitting together, and if the crack is too big, ditch it. If it's small enough, you might get some improvement by rotating the piston a little so the cracked portion bears on a different part of the pad.
Remember that there are no return springs in a disk brake. The only retraction is from the elasticity of the piston seal. When you push the brake, the seal is deformed a little, and when you let go, it tries to pull back to its normal shape. It's a delicate balance, easily upset. If you're too cheap to replace calipers (as I am for sure) it can be useful every once in a while to take the time to exercise them. The piston doesn't move very far at any one time in use. Pushing the piston back and forth a few times can help a lot to keep them from getting sticky. Just make sure you don't get overenthusiastic and pop the piston right out.