Having all of your eggs in one insurance basket could cause you a problem, IF you get a bad moving violation. If you get a DUI or Reckless, or something like that; your insurance carrier could deem you a "higher risk" on all of the vehicles that you insure through them. In turn, they could jack up your rates accordingly, or even drop you/some vehicles.
That's about the only downside.
The best upside aside from savings; is such that an Umbrella Policy works pretty well when you have everything under one roof.