I'm wondering whether the router allocates space even when its switched off. But when its off nobody can use it. But when its on you have access to it if nobody else is signing in.
Nah definitely not.
It doesn't make any difference if the shared hotspot is enabled or not, you always have full access to 100% of your sync speed/line rate.
It's designed to only allocate unused bandwidth to any 3rd party connecting to the hotspot.
If you are using your connection then it either doesn't let anyone connect to your hub or it lets them connect but they get allocated little or no bandwidth.
If you're on BT and have 1 of their hubs then give it a try. Connect to your own hub via the BT WiFi app and see what speeds you can get. Then start a big download on the home connection and repeat the test. It prioritizes the home user, as it should.
Unless you live within wifi range of somewhere members of the public are stationary (like a bus stop?) the chances of your hotspot ever being used are probably slim.
Apart from possibility a few pence a year additional power usage by the hub from the public hotspot being active, there are no negatives I can think of having it enabled.
I still disabled mine during the brief period I used my BT Smart Hub 2