Most profiles don’t have tone curves because they were created with software which was not capable of adding them and because we wanted RT’s image rendering to be accurate, not attractive. Newer ones do because we now use dcamprof which can add them, and because they make the image less “dull”, something which is a common complaint when one sees what the real raw image looks like for the first time, and because you can turn the curve off if you don’t like it*, so no harm done by including it.
* If you turn on the DCP curve, turn on the DCP look table. If you turn off the DCP curve, turn off the DCP look table.