One big advantage, on top of what Chris said above, is management experience, especially if compared with the competitors. Try Agony Meter if you need proof 🙂
Of course, as you mentioned, advantages of a dedicated management server are more obvious for larger environments. This is the main reason why Check Point is the leader for enterprise FWs for 22 years in the row, according to Gartner.
However, if you have a single FW and do not want to grow at all, standalone deployment (GMGT + GW in a single appliance) is also a good option.
Depending on your bandwidth requirements, Quantum Spark SMB appliances can also be an option. They can be locally managed through a WebUI, policy included.