Having some Crawlas shoot and others not effects the player's perception of them and will ultimately result in cheap kills, though. How is the player supposed to distingiush a shooting one from an ordinary one? It's not the frequency that's annoying (although granted it could be if overdone), it's the arbitrary manner in which they're fired. This could easily be circumvented by providing a visual discrepency or clue of some sort, but at this point the effort required far outweighs the need or demand for such a feature.
Crawlas are fine as they are, really, they can still provide a challenge when used correctly whilst still serving their basic purpose as low-level cannon-fodder, gun fire would just oversaturate the formula, not to mention levels would have to be edited and balance tweaked to accomadate such an attack.
TL;DR It's really not worth the effort.