I don't know if this will count as hard numbers, but here goes.
Last season in battle league, I posted all of the decks I used more than once here:
http://elementscommunity.org/forum/deck-compilations/the-dragon's-bl-decks-22013/The total number of dimshields used in all of those decks combined is...
I don't think that dimshield needs a nerf. However, if it were to be nerfed, my suggestion would be to change the 'prevents all damage for 3 turns' into a 5 damage shield for 3 turns (or 6 damage if upgraded). This would keep the massive temporary damage prevention theme of the card intact, but give more ways to get around it. Large creatures such as dragons, steam machines, and so on could still get past it, but it would still do basically the same thing it does now against smaller creatures. (I believe I suggested this once before or something very much like it, but it was buried in the fifty-odd pages of arguing here, though it may have been in chat that I suggested it)
Winning a league without using dimshield doesn't prove on its own that it doesn't need nerfing, but it does make a case for it. (maybe I'd have won more games if I had used it, for instance) I didn't actually even realize I hadn't used it at all, but I did know when preparing this reply that I hadn't used it very often. I never made the conscious decision not to use dimshield; I simply didn't design and play with any decks where I thought it would be advantageous to include it.
I will concede that in certain limited environments, dimshield is overpowered. One of the most famous AI4 grinders is simply just monoaether with dimshields and phase dragons/immortals, but that's because virtually no AI4 decks have a way to deal with that kind of deck. It's also overpowered in certain restricted pvp environments, such as some tourneys or other forum events. (I've heard complaints about its presence in War, for instance, but I haven't been in War since #5) When a fully unrestricted pvp metagame is considered, I don't think dimshield needs a nerf.