Are there any benefits to having multiple power armor frames?
Solution 1:
Yes
Companions can wear Power Armor. Therefore, keeping multiple Power Armors allows you to give your extra Power Armor to your companions.
Solution 2:
Companions can put on power armor if you direct them to it.
Also, if a settlement is under attack and there is unused power armor present, a defending settler will put it on automatically.
In either case, if you want the power armor back, talk to the wearer and there will be an option to ask them to remove it.