Just be sure to check out the reviews of the included hotels on www.tripadvisor.com because if they've put you in crappy hotels you will be sorry. But there IS some advantage in a package deal in that all of the details are taken care of for you...like train tickets, plan tickets and hotels. But like others have said, I like to plan my own trips, research what hotels I want, plan my own schedule with trains.
If you've got an open-jaw ticket with this package (into Rome and out of Venice) then that can add a few dollars more to your package. Are you arriving at a decent morning hour into Rome...in other words you're not landing at 5:00 pm are you? Because then you're entire first day of sightseeing is taken from you. Likewise if you're flying from Venice Marco Polo airport at 6:00 am then you're going to have to checkout of your hotel VERY VERY early (missing breakfast) and probably take an expensive water taxi to the airport.
There are pluses and minuses to consider.