Depending on their range, the 15 seconds is pittance for how much damage they could possibly do. On waves though? Eh
Right now, the Mk I stats:
Attack 90,000 Range 13,000 Reload 15 seconds
Health 1,050,000 Hull Polycrystal
Multipliers
10 Heavy 10 Ultraheavy 10 Structual 10 Turret
So, a single shot will pop any Mk I turret. It'll pop most Mk III turrets, for that matter.
Under a forcefield? The forcefield takes 900,000 damage, and 25 targets underneath take 56,250 damage. That's 2-5 shots against a forcefield to kill most Mk I turrets underneath. 3 shots to kill a Mk I triangle ship under a forcefield.
So, yeah, while the 15 second start up time may let you thin out the attacking wave, any ships left are going to wreck pure havok on your defenses. Add in the fact that Exowaves are usually led by something tough and speedy, and these things can be in your face and charging their shots before you know and before you've had time to deal with whatever the group-leader nasty is.
Thankfully, with 'only' 13,000 range, you should be able to close in a reasonable time. Unlike those annoying Bombards...
By the way, did the Spire Starship get a 100% attack buff while I wasn't looking? I thought it did 120,000 per shot, but when I just glanced at it in the latest build, it said 240,000 per shot. Didn't see anything in the patch notes, either.