I edit my patterns pretty thoroughly before publication and even ask friends to test them for me when they can. Even with the utmost scrutiny things can go wrong and we may miss something. With Novella, I tried to offer up three different sizes (in two different weights), but sizing can be very difficult. With Ravelry, if you discover an error you can send out a mass message to your buyers, and selling it in the shop wouldn't offer me that option.
So I opted for test knitters. And it was pretty great.
Test knitting allows you the opportunity to work out any (and most) kinks before publication, you can cover all sizes you'd like and you don't have to beg your friends to knit for you :) Plus the results can be pretty awesome.
Pattern testing also allowed me to see what my design, an all-over lace pattern, would look like in a variegated yarn. All in all, it's a thumbs up for pattern testing and now I'm ready for Sock Summit!