Here is my opinion on gimbal arm and cover cracks. I have sold, repaired, assembled and disassembled more than a hundred Mavics.
On some occasions, maybe 10% of the time, the unit arrives to the customer from the factory with a cracked or completely dislodged gimbal roll motor back cover. This is due to someone at the factory misaligning the gimbal clamp, forcing it on, and stress cracking the part. In all my repairs, I have seen only 1 that completely broke off, and all the others were just cracked laterally in the middle of the cover. On the other side of that cover is a pretty sturdy epoxied mesh, so just because you are seeing cracks on one side of the cover, doesnt necessarily mean it's going to crack any further because there is no stress on the part. However, if you continue to abuse it by using the stock crappy OEM gimbal clamp, it IS going to break eventually and you will have to send it in to get replaced so your roll motor wires are not exposed.
The other cracks that I am seeing are the ones near the screw holes at the base of the arms (covers). Those are a result of overtightening the screws at the factory, and the part itself is flimsy enough to crack. Those dont worry me either. If the part falls off, your bird is not going to fall from the sky, your video is not going to be affected, and the worst that can happen is the back of one of the motors will be exposed. It is mostly cosmetic.
IMO you should send your unit back to DJI if:
1. Your roll motor back cover has broken off and you can see wires
2. Any other cover of the gimbal has broken off
You might keep it if:
1. You notice cracks but are vigilant about preflighting your bird for further extension of the existing cracks. You can use a black magic marker to trace the existing cracks, if you see the crack continuing over additional flights past the end of the marker line, monitor it to see how far it goes.
2. You are comfortable repairing your own bird and are handy with epoxy or superglue
3. Nothing has broken off, and the cracks are microscopic.
If you have found cracks, take pictures of them now and save them just in case it gets worse and you do have to send the bird in.
My best advice is to throw the existing gimbal clamp into the trash and buy an aftermarket clamp that holds on to the lens instead of the fragile rear roll motor cover. Here is one by an MP approved vendor:
Dynamic 3D Solutions
If you insist on keeping the OEM clamp, check your bird for cracks on the gimbal over time. Also, its easier to install the OEM gimbal clamp when the bird is on its back.