Justin, the trim stop is helpful for holding narrow boards, but it has limitations. Consider, for example, using a
festool domino to join a narrow (horizontal) rail (say, 1-1/2" wide) with a (vertical) stile, where the rail butts into the stile and the outside edge of the rail is flush with the end of the stile (i.e. forming a corner). It would be nice to be able to register one of the Domino's pins on the end of the stile and outside edge of the rail, but the distance from the pin to the centre of the cutter is too great to be able to do that--the cutter would cut through the inside edge of the rail.
What you could do is mark the centreline of the mortise on both boards and use the trim stop to hold the rail when you cut its mortise. Because there is no registration available for both cuts, you would have to adjust the Domino to provide a little sideways wiggle-room in one of the mortises, and then align the joint manually.
If you were to instead use a spacer, you would have a registration surface on both boards, so you could cut a tight mortise in both boards. That would be faster too, as you would not have to mark the centerlines, or install and adjust the trim stop.
I can think of only a couple of situations where the trim stop provides registration for both boards being joined. In both cases the boards must be the same width. One is joining boards end-to-end, the other is joining the end of one board to the face of the second (at the edge of the latter). (For the latter, to cut into the face grain, the board would be placed vertically into the trim stop.) For these joints to be accurate, the boards would have to be centred precisely in the trim stop.
Note that the trim stop cannot be used for mitre joints.
I find that I can hold narrow boards securely when one face of the stock is pressed against the Domino's fence and another abuts a spacer. That is, I don't really don't need the trim stop for holding material in place. (The stock does need to be clamped down, of course.)
Cary