It may tie to player styles, too. I've had a few sessions where the momentum after the roll was over 10, so 4+ must spend. Biggest roll, 6 helpers, Computer, and every PC in specialty, with enough NPCs to zero out the difficulty. This was a torpedo attack on a Borg Cube... They got one free die, and used threat to get two more... (talent triggered, too, hence the threat spend) and thus wound up with, IIRC, 16 successes, for a 22 point momentum pool, and 16 must spend, on hitting the cube. They maxed out the number of hits, maxed the damage dice, rerolled the damage dice several times to get more damage out of said dice pool, and bought several conditions. When I finished the crits, the conditions were irrelevant... they popped it in one volley.