We have 16 bench spots in a 10 team league, and I MUCH prefer it that way. It not only promotes trading but also rewards those who draft well outside the first couple rounds. In some leagues, people get their first few starting slots filled and then cash it in knowing they can repair a bad bench with the waiver wire as the season moves on. It almost rewards people who draft poorly, start out with a bad record, and slide to the top of the waiver order.
Now, for most, 20 round drafts with room for 5 additional FA pickups is bit extreme - but as a general rule, I would prefer deeper benches. Either that or really short benches so that there are players available on waivers for everyone.
Just a personal preference.