I still play my Game Boy Pocket regularly, take it with me lots of places, the screen is still very usable.
In daylight (especially outdoors), you cannot normally see the backlight on a backlit LCD, or even frontlit ones. If it's something recent like the 3DS, the screen is so naturally dark that you have to crank up the backlight to maximum, that just kills the battery life. I especially love the AGS-001 (SP frontlit) and NTR-001 (DS Phat) for this reason, you can turn the lighting off for when it's completely unnecessary. I hate taking my 3DS out of the house, it's not ideal for daylight use.
Then there's the Game Boy Pocket and Color, they are very easy to see in most lighting conditions and have always been easy on batteries, even better if you like using rechargeable cells. It's not a big deal to me that they can't be played in the dark.
Eventually I'll have a Game Boy Light in the mail, it'll be neat to try it out after years of Game Boy Pocket and Color usage.
If the DMG counts as a guilty pleasure, then I refuse to feel guilty.