I'm of the impression that for web design, the difference between matte and glossy isn't as big of a deal as the colors don't necessarily need to be precisely calibrated. However, for film and especially photography/print, where the created medium is not the same as the finished medium, calibration is much more important.
So basically,
PC to PC viewing (web pages, pictures, etc)- calibration not as important
PC to print - colors will be off unless calibrated
PC to film (TV shows, DVD's etc) - colors will be off unless calibrated
Additionally, for web pages especially, sticking to the "web-safe" colors will do more for ensuring proper rendering than, say, monitor calibration or monitor finish (glossy vs. matte).