Well, banks charge credit card fees to vendors, so if you're a vendor, it might make a little more sense for you to accept a different medium of payment, such as Bitcoin. Assuming, of course, that it's widely used and that the volatility won't get in the way. You'll probably only save a few dollars per transaction, but that piles up rather quickly. That's a more than valid use case in my opinion.
Yeah but this kind of goes to my point that these same venues like Amazon could create their own digital currency making an alt coin, that applies their platform to the specific use of creating a universal alt coin specifically for dealing with online vendors, somewhat obsolete. Sure one could argue the mom and pop online retailers might find value in this alt coin, but I would assume they would probably just accept a widely used crypto like Bitcoin since it is more universally used.
There's also the fact that you won't have to deal with chargeback fraud, which is another pro.
For online retailers like Amazon or eBay I think this would be a negative factor with the amount of illegitimate 3rd party vendors. Also, not sure how customers would feel about this in general. Having their money held hostage unless the merchant deems them innocent. Sure from a legitimate vendor's point of view your synopsis is spot on, but customers will always think of the "what if's" when buying certain products, especially high priced items.