I Play in windowed mode generally, and often tab in and out of games while I work. So I can either do things like Farm the Auction houses in mmo's or craft stuff. Sometimes to look up locations for items drops and stuff. Its a rather common practice for Most MMO gamers to leave their browsers open all the time while they game.
Closeing some pages doesn't correctly leave them in the right state either. The general amount of low end traffic from stuff in the background isn't generally a problem, Nor is flash traffic for most stuff (Since i disable most of that with ad blocking and don't really frequent flash sites).
Overall Performance wise with i7 even at low speed you are unlikely to run into performance problems due to having multiple pages open inthe back ground. Generally with 2 or more cores this is pretty much the way things work. It's a habit i Developed close to 12 years ago when I started using Dual Processor graphics workstations to have large amounts of browsers and pictures open at one time for refrence work. Rarely have I ever seen it cause an actual performance problem outside of large amounts of memory usage.
Generally browers in the last 6 months are so have become alot more inflated and memory hungry due to the increased bulk due to security handling and incapsulation of tabs as their own entity instead of bleeding into the memory space of the other open tabs. Its part of the price you pay for security.
As for what i generally run as a default on my system even before opening a game, Steam, 10-20 open browser tabs. VMware Vsphere management console. Itunes/windows media player. Sometimes pandora desktop APP. I would consider this to be a pretty consistant Default amount of stuff for me.
