I'm with your friend. It's one more thing that your opponent has to deal with. And as 5263 said it's a fairly natural outgrowth of the modern swing paths. On high topspin shots you can get kind of an American Twist thing happening too where the ball kicks back in the opposite direction of the spin.
The side spin on slice can really mess with people, even really good players. Honestly on low slices if there's not quite a bit of side spin on the ball I don't see how you could be hitting with a modern slice technique.
There does need to be a good does of pace to make the spin effective.