Another way to look at it is this.
With a triggered drum tone, it grants the drummer a much more consistent bass drum sound live. Without them, one is subject to whatever bass drum mic the metal club has (if you're not a national touring artist), and how well the soundman can make you sound...which often isn't good. With a trigger, it is still an "input" to their mixing board as a microphone would be, but the tone coming out of the trigger module is already pretty close to sounding perfect, so the soundman doesn't have to tweak it much...if at all. Triggered drums work well too because they often can generate a "clickier" (read: more treble-laden) sound than the average miked bass drum. This helps as in many smaller clubs & theaters the subwoofers are usually skimped on first, so the bass drum loses presence in the mix. With the "clicky" tone, you actually hear that the bass drum(s) is going....it cuts through the wash of the rest of the mix.
And again, if the drummer can't play consisently, you just hear his mistakes more pronounced and clear...so it can be a great aid, but also a great hindrance if you're not up to snuff. It has made our drummer quite a bit better since he's bought them. Keeps you more "honest", because everybody hears what you're doing now much better.