I will sell the customer whatever he/she wants, but generally they will prefer a sewn hem. Would you want a pair of pants that were taped together? I don't care how "technically superior" a taped hem might be, a sewn hem is what most customers expect and are willing to pay for.
Most wholesalers will sew a hem for about 50¢/foot. That adds about $12 to a 4' x 8' banner. Another good reason to outsource banners. You would have to be making dozens of banners a day to afford a commercial sewing machine, the large sewing tables and shop space, and a trained operator. When I can buy a 4' x 8' 13 oz. digitally printed banner with sewn hems for $36.00 plus shipping...