Thread consumption is higher since the stitches lie diagonally across the fabric. The stress is not on a single line but apportioned across the span. Advance is an example of company that still believes in a straight stitch and a zigzag stitch for all seams. This maybe why they are also some of the most expensive gliders on the market. Other companies use a zigzag stitch selectively on line attach points and associated areas.
Bar tacks are a type of zigzag stitch.
Manufacturers only use small bar tacks to reinforce the most critical areas. Mylar panels on the leading edge is one particular area. Regardless of the type of stitching used, all seams should be even in length and thread tension. Incorrectly sewn gliders will have seams where the thread is loose or there is puckering of the material on one side but not the other. The large logos and color blocking on the top and bottom surfaces of many gliders can serve a hidden purpose.