If I see a NIN constantly doing DE on a longer fight then I just start using SP. Most "good" NINs will realize they don't have to DE since I always start each fight with SE combo and have it up pretty quickly, but in dungeons and other easy content it really isn't a big deal if both are overwriting slashing debuff since most things are shortlived in those cases. It is kind of nice though when doing massive pulls and you get down to one or two mobs and you already see that DE icon is on, but if it isn't then its not that big of a deal to just do eye yourself, or just continue to burn it down just as quickly.

Now in raid content, and the few times I have had a NIN (we don't have one in our static sadly) I just communicate that I'll be doing SE and there's no problem.