On the Canon 1Ds bodies there is a custom function that lets you set the brightness of the autofocus points. I find them a lot easier to see if set to the brightest setting. Are there any downsides of doing this. I guess the battery consumption would be slightly higher, but other than that are there any cons of doing this? Does it decrease the life of these lights at a higher rate maybe?
That's a new one on me.
But no, the battery life won't be noticeably shorter in standard use (via a quick search for "LED power drain") and LEDs like that will outlast the camera body (unless you start spending 100's on repairs to other parts of the body).
You personally could experience a slight loss of night vision if you're looking at the night sky or wildlife. And if you keep pressing the shutter down without your eye blocking that viewfinder then it would be a tiny bit more visible from elsewhere.
Other than that it's as Michael Clark 1 says in that it's to adjust to changing conditions.
The only downside might be that you might get a little more glare/light bleed from it bouncing off the focus screen and if shooting in low light, it might have more of an impact on your night vision. It's entirely a user preference thing though, there isn't really any advantage or disadvantage beyond using it the way you like.