The "Rematch Clause" is like every other stipulation or tool WWE has created to effect it's booking, they created it at one time or another to add intrigue or get them out of a jam & now it just exists whenever they choose to acknowlege it.
Just like every other "Rule" created by WWE, sometimes it works very well, sometimes they forget or don't care that they created it, & sometimes it just fails. Sometimes it can be useful in furthering a feud or adding the old competitor to the mix for a triple threat & sometimes a performer just "doesn't envoke their rematch clause" or they have the match on a TV taping or live event just to have it & move on. So it can really go both ways.
If I'm not mistaken Alberto & Cena actually traded the WWE title back & forth last year in a rematch clause match, even though I think Alberto's rematch was technically inside the Cell with Punk (who would have technically been envoking his rematch from Summerslam).
So it can happen sometimes but I get your point, it has become predictable. And while I somewhat agree, I can see why they keep the rule & i think it adds a slight shade of true competition & credibility to champions & the belt. So just like the foot on the ropes rule, it will get predictable & put on the backburner for awhile until they think of a reason they need to use it or some way to work it into storylines.