Several reasons. One, more stage coupling, two, the speakers being off axis will sound deeper to the bassist, and three, you don't have 2 of those 10" speakers pretty close to ear level making it hard to hear the other members in the band just to mention a few.
Actually, I don't see much need for an 810 cab unless your stage volume is really loud, which is fine if you play Yankee stadium or for outdoors gigs and you need the stage presence, or to avoid dealing with a 100lbs 410 cab. Other than that, It's usually better to let the PA do the heavy lifting so to speak. JMO