I had this exact same question yesterday at the LBS. We both agreed that the need to plug a tire was almost non-existant in our many thousands of miles of riding. More often you get a slashed sidewall or slashed tire, and you're not going to plug that anyways. So I just carry an extra tube on long rides.
If you're doing a lot of single track or something where you could get a decent puncture from a sharp sapling stump or a steam crossing sure. Though I have never gotten that big of puncture even riding my MTB.