Jailbreaking gives you the ability to get apps outside the App Store ("Market") and ability to get many free apps...
And it's 100x easier then flashing your phone with a ROM.
Flashing a ROM is usually extremely easy. Especially if the phone you are rooting has been out for a while. There are even apps that will root your phone. ROM Manager will install the custom recovery and ROM. You act like it is difficult. My T-Mobile G2 is now easier to root and flash a custom ROM/recovery then it is to jailbreak an Iphone.
As was mentioned before, on a stock Android phone you can install apps outside of the market and install pirated apps. Android gives you more freedom to do what you want with the OS.