Perhaps Dini squandered that potential (I disagree on that count, but I can understand your point), but that potential was still there, and I just wish another writer could have taken that complex potential of that Riddler one step further rather than reverting him back to one-note evil, albeit a brilliant one-note evil.
But then, this is the (apparently) necessary evil of superhero comics: everything must return to status quo eventually. Sometimes, I take comfort in that knowledge (like Hal Jordan coming back, and the eventual return of Ted Kord). Other times, like this (or FACE THE FACE, which I hate for very similar reasons), it makes me wonder why I bother following them at all if character development matters so little.
I'm not saying we couldn't have villain!Riddler back. I just wish he'd get there through development, rather than a sudden reversal ala Guy Gardner (right down to "I'm BAAAAAACK!" which only worked in JLI because that book was a comedy).