Mudi has a battery. If you leave it always on, the battery is always charged. This is not good for the battery. This is the same for smartphones.
For other routers without battery, of course you can leave it always on.
BTW, we are improving our MiFi design so that even with battery it can always be powered on. This needs to design a circuit to protection, cycle the battery so it is more complicated and target at more commercial usage.