There is nothing wrong in having this kind of service and I think I was some service like that in the past that performs some free acceleration service up to certain point and then charge fee for those above the benchmark. You can then make it a more bigger one so we can patronize it.
I guess that it might be more profitable in the shortrun to simply mine "normally" but that in the longrun offering a transaction service for a low fixed fee could be what btc needs to brake the trend of increasing fees. You could potentially offer a lowbudget alternative with lower direct profits but a stable amount of clients willing to pay for each transfer and always to you. Not sure if it would work in real life, but thats how I imagine it.