Captain Keogh agreed that he might need more fire power to rescue Sisko, determine Dominion intentions and strengths, then get out of there. He had O'Brien refit the Runabouts just for that.
It seemed to me that Keogh was just humoring our DS9 heroes, letting them symbolically help with rescuing their commanding officer and his family. He didn't put any faith on the runabouts actually being useful in combat.
And I'm not sure if they were useful in the end. Sure, Keogh was able to send one of them to fetch Sisko and Quark - but he would have had that one (Sisko's original one) available in any case, and he could have sent one of his own shuttlecraft to do it if he didn't stumble on Sisko's craft.
And whether the DS9 runabouts took any pressure off the Odyssey, only the Jem'Hadar really know. They might have been going easy on the runabouts in order to leave witnesses for the inevitable destruction of the big ship...
Timo Saloniemi