What cost? Writing a number down once a week? I think a lot of groups have stopped using them because their value is not intuitively obvious and their cost is. But I do think it creates a subtly but significantly better player experience, at only a minor bookkeeping cost to the DM.
Figuring out how much to award, for what, when, tracking those activities in-game, and then adding it all up.
If you're not thinking about how much to award, for what, and when, you're not rewarding anything. You're just picking a random number. I have an otherwise great dm who does this, and it's frankly
worse than not giving xp because I get a weekly disappointment, because even the sense of reward I did get once is now soured, because that reward was removed.
Obviously experience points won’t fix a game that isn’t otherwise fun. They also won’t take the fun away from a game that is. What they will do is make a subtly, but significantly, more psychologically rewarding experience. But yes, the game still needs to be fun. Figured that went without saying.
I can tell you for certain that experience points do not always significantly improve fun. I've seen them ruin fun just as often, either by encouraging un-fun gameplay (just kill everything, only follow the plot, skip rp to go for more points, etc) or create tension and feelings of favoritism, or just be so arbitrary as to lose any sense of reward.
The idea that any game would be better by adding experience points is a big claim, and one not borne out by any evidence that I can see. If anything, the number of people who
drop xp is pretty good evidence that it isn't true.
And if xp is only helpful
if used well - that was my initial point. It's not always good because it can be used badly. Using the tool well is possible, but a skill, and badly used tools, or tools built for one purpose being used for another incompatible purpose, are bad.