Brenda - I'm of the 'Ain't broke, don't fix it' school. I'd never used one on my Janome and didn't feel the need. However, when I got my Pfaff QE4, I got one included in the deal and I was glad I did! Reviews on the Pfaff had also highlighted that it 'ate' fabric which is why they probably included it.
I wouldn't buy one unless you really needed it. I don't have one on my Pfaff Passport I use for class and don't see me buying one any time soon (naturally the other one doesn't fit, lol).