There are NO diminishing returns on this immobilization. Used by an otherwise competent arena team this should be difficult or impossible to counter-10 seconds is a long time in arena.

No one knows yet whether this set will make it into the new season on the 4th, but it would be a good idea to be prepared. Historically something like this has been ignored by Blizzard until there is a significant number of complaints, then the opportunity is nerfed without any retrospective action against players. So this leaves a relatively small window of opportunity.

EDIT: Respondents below mentioned a recent hotfix by Blizzard which was supposed to stop this sort of exploit happening.

However, the set bonus, which is what we were actually interested in, still procs regularly against high-level targets.

Interestingly the effect associated with "Fang Of The Crystal Spider" is clearly labelled on the item's tooltip as being only valid for level 60 and under, and did not proc despite extensive testing. So Blizzard made an attempt to fix these items but obviously trying to do this manually will, at least initially, miss a lot of powerful procs.

EDIT EDIT: Wow this thing rocks in pvp.