Even if it is 90% people abusing the system, at least those who are deaf still have a really convenient communication option. Even as a non-deaf citizen, having no deaf friends or relatives, that is worth my tax dollar. The fact that the service is available at all is quite an accomplishment, as inefficient as it may be.
Look at any recycling service, for example, it is inefficient and wasteful, but to the environment it is helpful in the long run.
The same goes for the internet, milk, subsidized crops, national parks, landfills, etc. It is something we must accept as a society in turn for a greater cause.