Pros/cons to turning off cable modem

Provided that you don't have your computers exposed through the DMZ or open ports of the router, and the router has it's web-administration disabled then you've already mitigated most of the risk of leaving the router + cable modem on when not in use. If your computer is infected with viruses or other ilk then it will probably do it's dirty deeds whenever it's connected to the internet--even if you're using it.

Keep in mind that OS's and software also try to keep themselves up to date, and often times the default schedule for this maintenance work is during off hours. Disconnecting the machines from the net during the night may delay receiving updates, and at the least will likely mean that you're using the machine while it's installing these updates.

My two cents is that unplugging them might save a few dollars a year (I doubt much more) but will likely not do too much to stop mal-ware from doing it's deed if your machine gets infected and will almost certainly be a hassle for you--so long story short, my vote is to leave them connected.

There is no benefit to leaving it on other than not having to turn it on later (which to some people is a major hassle), unless you have scheduled things to run at night.