I don't know why people assume that because quest based leveling is being added that other forms of leveling will not exist.

It may be that quest based leveling is faster solo while party leveling is faster with mobs. It may be the opposite. It may be that quests are only slightly faster, thus giving people an option when they can kill extra fast.

Keep in mind that we need to be able to accumulate experience after all quests are finished in order to spiritbond items at endgame... so killing mobs will still grant realistic exp.