The only times I've ever used the back switch is when I'm putting it away for a while (i.e. a month or so). The button on the front essentially puts the PS2 into "standby" mode, but keeps an inkling of power running through the system.
Here's how I see it: Picture a light bulb. Lasts for quite a while, yeah? Thousands of hours, even if you leave it turned on 24/7. But start flipping it on and off repeatedly, and it'll blow in short order. When power is reintroduced to an electrical system, there's always a small surge that can cause a slight "shock" to the components (i.e. the filament in a light bulb). This surge is so small as to be virtually unmeasurable, but it's there.. and over time, can add to the wear and tear of the components in the system, which may cause it to break down before it's time. By leaving the main power on, and simply putting it in "standby" mode, it prevents this.
I saw something similar concerning computers not too long ago. The experts said that it's actually better for the hardware in your PC if you simply leave it running all the time, rather than turning it on and off repeatedly, due to the stress this causes the components.
Granted, this isn't something you really need to worry about.. turning it off with the back switch isn't going to make your PS2 explode, but the cumulative effects of using it will eventually shorten it's lifespan.
So, going back to my first sentence.. I only turn off the main power when I'm putting it into storage (i.e. back under the entertainment center), or if there's an especially nasty lightning storm overhead. Otherwise, I just use the front button.