At this point, Janeway knows next to nothing about the Borg. Not unlike Picard, who had much more on-hand experience with them. Sure, by the end of the series it was the other way around, but at the time of Scorpion... no. Picard would know better then to trust the Borg, and would never even consider an alliance. He would find other ways to get to the core of the problem, perhaps even open up a dialogue with 8472.
I wonder if Janeway, knowing what she learned of the Borg by the end of the series, would be so open to the idea of an alliance with the Borg.