if you listen to the developer commentary its clear that some of those were nescessary to get the game principle across to ame... thick people
like the one very early on in an l shaped room with 2 pits you cant cross without the portals and youve only got the ability to open one
actually one of the guys in an irc channel i was in at the time took quite a while to figure out that the portals arent one way which the level is all about so those early levels do serve a genuine purpose for some