Yes you can leave it in and it will charge it up until fully charged, and then the unit will not charge any further. In fact, Garmin recommends that you have it plugged in all the time while you're driving, to keep the unit fully charged.
Newer Garmin units have larger screens, faster processors, deal with bigger map databases. They are set up to be plugged in. 25xx series dispensed with pedestrian mode, presumably because of the shorter battery run times.