XP is needed to progress, unlock things etc. Also for when the BP is released. Why would people play where xp and progression will not be awarded? I mean yeah its also about having fun, but people want that feeling of progression too, and to unlock the few goods it offers, and for when the BP comes out.
Tons of people dont play Portal because of the restrictions, i have seen it everywhere. Also seen whole communities that dont want to play it because of this. They wanted to build proper community servers for the members but no one will play it. Everyone is pretty much forced into AOW because that is the only way to get proper xp and progression. A lot of people (including myself) bought BF2042 for Portal.
XP and prog has been in BF games for a long time now and is the reward for the activities you do. Just because you might not be bothered about getting it, a lot more people are, hence one of the main reasons as to why Portal is dead.