It depends. There's nothing wrong at all with doing that if it fits. I think it's more of a matter of, if you take an uninteresting unoriginal approach to the vocals, they'll sound boring no matter what technique you use.
Frank Zappa doubled instrumental lines with vocals ALL THE TIME, and he surely couldn't be considered unorginal.
Just think about what you want to stick out in the person's mind. Is there a very strong melody in the instrumentation that you want to reinforce? If not, maybe you should create a strong melody else where, namely the vocals. I find that simple, or overdone chord progressions sound really boring when doing this as well, make sure it's not the standard progression everyone has heard a million times. But really, like anything, it's not good to overuse any one certain technique. If all you did was follow the melody, it WOULD come out boring, and likewise if you try to do something different with the vocals just for the sake of it, it could wreck the "catchy-ness" of the track as well. Just mix it up, and think of the voice as just another instrument, instead of THE VOICE, when you're first crafting the parts (It may work for you, it helps me sometimes to view it that way.) =) But anyway, write what you feel, not what you think people want you to write. There are no rules, and people make huge hits, and very meaningful music all the time by basically "destroying music". You know what I mean?