Rogues are one of the most versatile classes. Rogues can be designed to do many different things. They do not have to be evil at all. Think of a rogue who focuses on conversation skills like diplomacy and sense motive. They have many skill points so they make great party spokespeople. You can make a rogue who is Neutral good who is a scout type character. You can make a rogue who focuses on combat - mix in a level or two of fighter and you have a very effective flanking "attack of opportunity" killing maching.
Rogues are certainly not necessary - you can be successful in this game with or without every character class - but I don't adventure without one. IMHO they are too flexible and interesting to leave out...
