What convinced you? I don't see it. The user is using SSH, if they can't pass a -p option (or type it in a GUI) to their SSH client they won't be able to do much with the shell they're getting either?
And, sure, you can add a -p option. But if you have 20 VMs (which is how many come with their basic plan) you'd have to remember all the different port numbers.
(I'm not in the target market for their service.)
You also can't really use the public hostname for this, can you. Unless you do really complex DNS trickery, you can only return one (set of) IP address for a given name. It would thus need to be the same IP address for everyone. Which works only as long as 2 users don't have overlap in the VMs they want to access…
(I guess they can run a solver and try to make it work for as long as possible, including reassigning IPs… but it'll hit a wall at some point?)