I think they are ok at the start of the game or it's like FF6 where you go somewhere you aren't mean to ( the Guardian). At the start it can show the power of your foe, or if there's some other reason like they got a super magical item or they are way more technologically advanced than you, unbeatable boss battles in those situations can help give you the feeling of their power. Like Wolf kanno said though, when it isn't pretty obvious and you waste a bunch of items and stuff it sucks!

Damnit Christmas strikes again! I've already commented in this thread 6 years ago!

The pikachu in Pokemon colosseum is still my favourite example of it though, the fact the battle is purely 2 AI fighting but somehow they still made it the Old man can win when they could have easily just scripted the fight.