You can still get bounce back from heavy blowing rain that lands under the edge of the fly which then soaks the inner tent walls if they are absorbant.
I think it's also useful having that seam, between groundsheet and sidewalls - particularly if they are part mesh - above ground level to reduce abrasion to that join too.
It wouldn't be an absolute deal breaker for me, if everything else about a shelter was perfect, but I think bathtub groundsheets are that way generally for good reason.
Good venting is also pretty key in a tent too imo.
Nobody wants to be cooped up in a soggy coffin.