It's always a good idea to have an accurate baseline. There will be variations in monitors in here and there, but they will only amplify issues if the baseline isn't as close to accurate as possible. Also, if you ever send a picture to a photo lab to be printed, chances are their printers will be very accurate, and so your monitor needs to be as well.
Also, many smartphones now (including all iPhones, and the iPad) have IPS displays. They are sharp, high resolution, and color accurate (to a degree). This increases the chances that an image adjusted on an uncalibrated monitor would render poorly. Also, if your monitor is displaying a stop or so underexposed, you could find yourself compensation and OVER exposing your images in your camera to match your monitor, when the monitor really should be matching the actual image!
Just my $0.02. Tools are nice but you can ballpark calibrate any monitor in a matter of minutes right from within Mac OS or Windows 7. It's not going to give a pro level result but it'll get you close, and I don't see any reason not to do it. It might take some pondering before investing in a tool, or purchasing a new monitor, but to spend 5 minutes with a built in color profile / monitor calibration setting in your OS and double checking it with a color calibration website? Why not! Can't think of a good reason NOT to.