It is for a rogue, not so much for a bard
You can check
http://wiki.project1999.com/Skill_Pick_Lock for the list of doors which can be locked and the skill required to do so.
As you can see, Bard won't be able to pick lock many doors. In addition, some locked doors don't have a key you can find to open them, so you will definitely need a rogue to open them (CoM, Sebilis, Chardok are coming to my mind right now )
CoM : opens doors to shaman/magician epic mobs mainly
Sebilis: opens door to crypt in which some nice loot can be found and also opens door leading to trakanon.
Chardok: opens doors to royals in which nice loot can be found as well as epic mobs