When you say "Platform agnostic" I assume you are referring to MMS, which the telcos (in my neck of the woods at least) like to charge extremely exciting amounts for the privilege of using.
If MMS was not so ludicrously expensive (even more so than SMS, egads!) then I would agree that Thrutu is pointless. But while MMS is unattractively priced, then I think it is more than reasonable to expect that less "platform agnostic" alternatives pop up to provide more reasonably priced options for the average punter. If my boss wants me to send him pictures of my cat, then he has two choices. Pay the standard MMS rates, or get a phone that supports a less expensive protocol.
I think the main point is that Thrutu is not aimed at enterprise types, it is aimed at Mr and Mrs Sixpack, who would like to be able to send each other pictures of their cats without discovering that their bill at the end of the month is so exorbidant that it leaves them struggling to pay for groceries until the next pay cycle.
Platform agnostic protocols are all well and all, but when the price is artificially jacked up to the point that they become unpalatable then you have to expect that alternatives will emerge.