If that were the case, RACs would only be capable of firing at 4+ rounds/turn. The very fact that they can fire anything from 1-6 means they were designed solely to put out a large volume of fire. It's just sill to be firing off 4-5 rounds/turn at high TNs where odds are none are going to hit. Take a speculative shot or two? Sure. But don't fire 4-6 rounds on, say, 11s.
Yeah, its kind of mad to accept a high risk of jamming if you're hitting on an 11 or 12.
That said, if you drill down further in to how the mechanics operate, there's a bit more to it. Basically there's no point to either 2 shots or 4 - 2 shots is delivering less expected damage than 3 shots at the same chance of jamming, and 4 shots is delivering less expected damage than 5 shots at the same chance of jamming.
So that gives us the options of 1 shot, 3 shots, 5 shots or 6 shots.
1 shot is an okay option if it's a long shot and you're being very conservative on jamming, or if you're running low on ammo (as you lose no shots on the cluster hits table it's the most efficient method firing)... but you shouldn't be doing this very often, or you might as well just take a light autocannon.
3 shots is basically the default for long shots - the risk of jamming is very low (1/36) and you can still expect about two hits.
5 shots is the standard basically. The risk of jamming is much higher (3/36), but you can now expect about 3.17 hits a turn. This is basically how the RAC should work - get to decent target numbers and just grind out 5 shots every turn until you jam, run out of ammo, or your opponent explodes.
6 shots adds only more damage still, jumping to an expected four hits, but the risk of jamming jumps considerably, doubling to 6/36. It's an option if you've got a near certain hit and you're looking to max out damage ASAP, no matter the risk.