Bubbles in water heater system
This is, in my experience, nearly always an indication of leaks in the system, so make-up water is being added by the boiler feeder (adding new dissolved air) or, if the boiler feeder is shut off, air gets in after water leaks out. In a system without leaks, once the dissolved air introduced with new water is removed there should be no further need to bleed it.
Apart from the inconvenience of having it stop working, the constant addition of new water with dissolved air also violates the assumption that when the system is sealed there's no more oxygen added to promote rusting of the iron components once it's either bled out or rusted a little.