Best not to leave the battery plumbed in if the car is left for more than a couple of weeks. It's not an old wives' tale, it really does reduce the life of the battery. My brother managed to muller a year old Exide battery recently like that.
One problem a critically discharged battery can give or even just removing a battery, is throwing out the security rolling codes between the EWS and DME. Again, not an old wives' tale. That one happened to me last week when I borrowed the battery for a project car.
You can get smart battery chargers now that stop charging when the battery is fully charged, but monitor it, and top it up if it discharges. The make of one I have is CTEK. You can leave it plugged in all the time. (except of course if you want to drive the car!)...

Bonzo.
