Be wary of anyone asking to be added to your insurance policy as "additional insured" carte blanche. Competent insurance companies and brokers will insist on a phrase consistent with "in so far as the carriers operations ...". Otherwise adding someone simply as additional insured means you will be insuring against all of the additional insured's liabilities.
It's not a bad idea for load brokers to foster and facilitate communication, but for the most part, load brokers should stay out of the claims process. Typically what happens between a carrier and a shipper vis-a-vis claims is the shipper will send an intent to claim, the carrier will investigate and respond, and the shipper will send an invoice for the damage. Where the claim goes after this point is entirely dependent on the carrier's response. If the carrier accepts the claim, the carrier (or their insurance) will pay the invoice. If the carrier denies the claim then the shipper will either accept that, or begin legal proceedings.
BUT
Carriers ... don't think you can just deny a claim when you are at fault, and force the shipper to take you to court. Inevitably it will come out in court that you were at fault, and not only will you have to pay the claim, you will have to pay punitive damages, and you will have to pay the shipper's lawyers. I am almost 1,000% certain no carrier wants to pay Loblaws' lawyer bill !!!
Shippers ... don't think you can submit an overvalued invoice in the hope that you can use the carrier's insurance as a cash cow. If the carrier makes a fair offer, and you deny it, and take the carrier to court, you will lose altogether, and the court will make you pay the carrier's legal bills. You ever seen a carrier's insurance company legal bills? These guys charge for the period at the end of a sentence. Trust me, it's cheaper to pay Loblaws' legal bill.
ANYWAYS
Where load brokers can be of the utmost value to their shipper clients, vis-a-vis claims, is as an advisor. If I were to hazard a guess, I would suggest fully 75% of shippers out there have no idea how to properly handle a cargo claim, and you can bet your bottom dollar the carrier is not going to help them out, so brokers who know their way through the claims process can become invaluable to shippers.