If you don't do it, someone else will... In our case, if you do swap in one direction and nobody does the opposite, the HBD stabilizer will be the "other" side and balance out the HBD price...
The "arbitrage" part will be done by HBDstabilizer, if it isn't done by a specific user...
It certainly does its part.
But I've often had a different partner in the swaps on the internal market as I was buying HIVE, often other arbitragers but also what seemed to be regular people too.