Well, quite. Seam taping on a jacket malkes a lot of sense: the seams are in constant motion being rubbed this way and that and that will work water through them. But a well made seam in a tensioned sheet needn't leak, as Saunders and Hilleberg have been proving for over 30 years.
If you're that paranoid some seam sealer will do the job, weigh less and not heat-age the base fabric when you stick it on.
