Being ambidextrous at pool is not a horrible thing, regardless of what game you play. Why would being ambidextrous in 9-ball be more important than in straight pool? I would argue that the bigger the table, the more useful being ambidextrous would be.
If you practice opposite handed, then you better shoot opposite handed every time an opposite handed shot comes up, or the practice is a waste of time. If you're playing a big match/set/tournament and a shot comes up that you could either shoot opposite handed or use a bridge, what do you do? If you think, "This is a really important shot, I better use the bridge cuz I don't want to miss with my opposite hand", then you wasted a whole bunch of time practicing opposite handed.
Personally, I'd say practice opposite handed a little bit so you feel comfortable shoot easy shots that don't require shape. That's it, don't waste any more time on it. Save the extra time for practicing to get better with your normal hand.
On the other hand (no pun intended!), if you ever foresee a "I'll play you opposite handed" scenario in your future, then practice opposite handed!