Lydia never sleeps in that bed anyway so you aren't missing anything. I even decorated her room with a Dibella statue but she didn't take the hint.
Just give her a chair of her own. I moved the one in the west corner of the loft, next to the cupboard, into her room and placed it next to the nightstand. She promptly sat down and stayed there. I can even close the door on her.

The other problem with the housecarl beds is that the priority of the guard part of the sandbox program is higher than sleep, so they never have the opportunity to use them. In any event, Lydia wouldn't be able to use her bed as-is, since it's the wrong model. The entry is on the wrong side. Or it could be turned 180°.
Owned-bed trivia: Did you know that the bed upstairs at Katla's Farm (house) near Solitude is owned by Madesi, the Argonian jeweler of Riften?
