For what purpose? Why not just use the QT client? The RPC interface is much slower than the QT client and so any client built on RPC will be slow. Even if there are such customers (never heard of anyone), who will use it?
If „we“ had the same mind set like your posting here, then bitcoin could never exist. Imagine people raised the same questions:
Bitcoin - what purpose? We have Dollars
Bitcoin - why use on a PC? I can send via my bank or PayPal
Bitcoin - why is it slow? It can only do 7 tx/ second - VISA can do 50.000s
Bitcoin - who will use it? Only nerds would do this...
Do you want to be in the same group?
Be open minded, and don‘t (implicitly) judge someone else’s request, cause your imagination might be limited...
Here an example, I discussed on a meeting last recently: the person was running a bitcoin daemon on a raspberry pi, in a headless mode, connected via ssh. And he was wondering, how to remotely connect to the device from a „powerful“ system with a graphical interface. He was unfamiliar with the command line shell. So he looked for something like a web app, that embeds the RPC calls, so he can do a minimum set of commands. And as you see, speed would not be an issue at all!
There are always things beyond our imagination.