The ability to write a custom WebUI is extremely powerful. The lack of a deployment option makes it very difficult to implement and test efficiently in an SDO environment.
Having to submit an SDO ticket for deployment is cumbersome both for clients and (I would imagine) SDO. A client with 4 environments would have to submit 4 tickets to SDO to deploy a single customization (you could do them all at once, but that sort of defeats the purpose of having a test environment pipeline).
Find a bug in one of those deployments? Better fix it and submit another ticket to SDO so that it can be tested. This is a LOT of added time/overhead for something that in a self-hosted scenario would be a drag and drop.
As CRM switches over to WebShell exclusively and the cloud hosting side of your business expands, this omission in the deployment pipeline will only become more glaring. The ability to self-upload customization specs and DLLs already exists because it was ported over from ClickOnce. PLEASE embrace the move to a Web based application and give us the ability to deploy UIModel files.