Either keep firing or run, don't just sit about in the same location talking about what you're gonna do, sheesh.
I think the justification is that Picard feels the need to show Q that they're ready for anything. It does kill the tension completely, though, and it's a pretty thin rationalisation. It's clear almost from the start that the Enterprise is completely outgunned and even if Picard is trying to show off, surely someone else would at least suggest
just high-tailing it.
Picard had thought they had damaged the Cube enough to not have to worry about it. It was Picard simply being being stupidly arrogant and overconfident, exactly what Q accused humans of being earlier in the episode.