Salac Berry/Belly Drum is really only used on things that also plan on using Substitute, to reliably and controllably cut off exactly 1/4 of their health. The idea is that while the opponent probably spends a turn switching to their best matchup, you put up a Substitute, then next turn they usually can't do better than breaking your sub while you Drum up. Between those two moves, you've managed to lose exactly 3/4 of your health and get into berry range, allowing you to take the speed boost and hopefully mow down everyone else.
In theory you could use it on someone with Gluttony so that Drum alone is enough to trip the berry, no other moves needed. But the only Gluttony + Belly Drum users are Linoone and Snorlax; Linoone prefers using Extreme Speed which doesn't care much about anyone's speed stat, while Snorlax is so slow that a +1 from the Salac Berry isn't likely to change anything.
