In reply to Sam W:
In the days before taped seams the seams were double folded/stitched lap seams (or something like that), i.e. each end of the fabric being sewn was folded and interlocked with the other before being sewn with two rows of stitching.
This is an inherantly very strong seam and is still used on silnylon tents because you can't tape the seams.
Once seam tape came along, manufacturers ditched the expensive/fiddly seams and just did a single straight line of running stitch. This is inherantly quite weak but they get away with it because the tape glues the two sides of fabric together (as well as waterproofing the stitch holes).
If you just stick sealant on this type of seam after the tape has peeled off you have a very weak seam with stitch holes which will gape under stress and still let some water in.
You may get away with this on a happy camping tent used in benign conditions but not on anything used when the weather gets properly bad. You would need to use some kind of self adhesive tape on the seams for this, like spinnaker nylon tape.