I've not found that a problem, but I came from a 12-inch iBook G4.
I also have an eMac with a 17-inch display, and I rarely use it. I'm almost always on the MacBook.
I always have multiple applications open at once, and they're easily accessible via the Dock. Usually Safari is running in front of everything else for me, but you could reduce the size of the window if you wanted to view another app simultaneously. I use Stickies a lot, a Mac application that comes with the OS and simulates Post-It notes on the display. You have the option of having "floating" notes that will stay above other applications, and they "Window Shade" up so that only a title bar shows on the display. Command + h hides every application except the foremost one. In my opinion, it's easy to find ways to optimize your work flow that more than compensate for the smaller screen.
However, size isn't the only issue here. The resolution of the 15-inch is 1440x900 compared to the 1280x800 of the 13-inch model. If you need (or want) the greater resolution, that could be the deciding factor for you.
Whatever you decide, I don't think you'll have any regrets because you'll be too busy enjoying your Mac to worry about what you didn't get.
Good luck with it!