If you can in deed endlessly hover, why devote any
internal space to those four legs?
Technically, if you're hovering (regardless if the legs are touching the ground) you haven't actually "landed."
Another reason to have the ability to separate would be, instead of the saucer needing to get away from the lower section (i.e. possible warp core breach), there could be occasions when you wanted the lower to be able to get away from the saucer.
Contamination, boarding party, irreparable damage.
The lower would give you (the Captain) a place to withdraw your crew too.
First Officer: "The entire crew is in the lower section, but the boarders now control the saucer section, what shall we do Captain?"
Captain: "Eject the warp core!"
First Officer: "Huh?"
Captain: "I mean, lock out the saucer's computers and separate the ship."