I bought my 6600 new and the straight stitch plate was not part of the "included" items. I bought it after I started having trouble with the machine eating the fabric. Then I realized it was kind of a pain to have to replace the plate everytime I wanted to piece and it worked, but wasn't perfect. I traded in the 6600 within 8 months for the 7700 and don't have that problem with the 7700.
I'd say go for it, can't hurt to give it a shot.