I was just going by the Halo 4 visual guide.
"After an initial assault they are deployed en masse to mop up."
I was thinking they'd be useful at the end, when all the ships are waiting for their second or third volley and are pretty heavily damaged. Having fresh ships instantly come in and help would be more useful. Plus with the targeting problems of SotP (most ships will target one ship,unleashing more firepower then is needed) their shots would probably be wasted in the initial volley anyway.
Yeah, I was going for a more of a lore naval battle when making the post. However I'd still think thy would be most useful at the start for much of the same reasons I said before. You said that Sins targeting selection is pretty bad and I agree being a long time player myself (of Sins in general) at the same time you won't have very many heavily damaged ships for the same reason, maybe a capital or 2. Targeting will take out all your frigates first then on your cruisers type ships then capital ships (mostly because frigates are up front while cruisers types stay just a little farther behind) So you want the Stridents in the fight at first volley to both add a bit more damage (destroying covie ships sooner than before) and soaking up more damage from other ships as well (when the stridents are targeted themselves)