What is a gimmick-instrument? it's when a band uses an instrument that no other well-known band uses.

now i'm thinking of In Extremo with the bagpipes and Alestorm with the accordions(though if you know more such bands please tell). what do you think of them using such instruments in their music?

personally i think it's a great thing, it gives the music some kind of a signature sound, like you know which band someone's talking about when you hear something like "the opener was this really cheesy band, i don't remember the name cause i was insanely drunk, but they used lots of accordions from the keyboard and i had outrageous amounts of fun", of course it's valid if they didn't lift that off from some other band of course

Alestorm don't use accordions. Just synths.

Turisas however, once had 2 accordion players.
Eluveitie have a hurdy-gurdy and several kinds of whistles and pipes.
Korpiklaani have violins, bagpipes and accordions.

And so on and so forth. The traditional instruments work very well in the folky subgenre, and I don't think they're gimmicky really.

Using such instruments is cool. I like the idea. I think In Extremo and Orphaned Land are the best examples of a successful usage of those. But I don't like it all the time.. There are many bands that use violin or cello on their music but they're still boring. Or as we speak about In Extremo that use bagpipes, there other bands that use the same instrument but they're not able to make an excellent piece of music. So I voted for "whatever makes a band sound good".

Its like any other instrument. Used well it will sound great, but if your writing is crap *coughKorpiklaanicough* then it is just a gimmick to make up for lack of song writing ability.

Agree with Dr. Rock.

Negura Bunget made fantastic use of instruments that are not traditionally associated with metal on "Om" - really made the album breathtaking.

Others, not so much. In some, i think it's part of a definite image push rather than for primarily artistic means. It shows in the overall music quality.

Bands can use whatever instruments they like as long as it adds to the music and/or sounds good

I like the use of that instruments but I think some bands abuse and make the sound a little disadjusted.

Negura Bunget is a good example how to add gimmick-instrument right.

Gimmick instruments eh? I think that's absurd, and don't imagine a band throwing in an irregular instrument just to get attention. Not to say it's never happened, I just think the unconventional instruments that may have been used in metal were used poorly or distastefuly by said musicians.