The main reason the Hobbs is used by commercial hirers, is that it allows them to advertise a lower "headline" hourly rate.
In my aeroplane, the tacho is about 20% behind the Hobbs; they both started at zero when the aircraft was flown for the first time.
So, to get the same revenue, the hourly rate would be about 20% higher if using tacho. That may amount to the same cost to the user, but it would be commercially less attractive to advertise the higher rate.
Putting an airspeed switch in the Hobbs circuit is relatively cheap and easy, and makes accurate log keeping easier, but unattractive for the same reasons as using the tacho.
I have always used tacho for group owned aircraft, as it's a fairer way of apportioning costs.