If that tab is fused, the failure of that fuse would indded cause you to lose whatever is on that line. As long as the fuse we are talking about is of higher amperage than the individual fuses for the components it should not fail. The lower amperage one would instead. The "IGN" tab will only be lit when the key is on and you won't have your power seats with the key off. If it were mine I may source the seats with full time power(in line fuse off the hot side of the box) but it's not a big deal.