There's a lot of text being typed up about Matisyahu and other Jewish Rock/Reggae/Whatever musicians and the appropriateness of their playing in Rock Clubs/Bars.
I'm not going to address the issue of teenagers, frum or non-frum, at these events. These kids don't belong in clubs that serve alcohol and are for 21 and over anyway. That's an issue for the parents and the club owners.
As a musician who has performed in rock clubs (both with Jewish and non-Jewish Bands), I'd like to give some insight as to why Piamenta, Blue Fringe, Soulfarm, etc. seek out such venues. It's not about kiruv, or making a point, or making a Kiddush Hashem, or making money (definitely not), or about trying to be a rebel.
It's about the music. That's it.
The kind of music that these bands play is best heard in this setting. What all these acts have in common is volume. This style of music is loud. You can't get the energy you need from a hip-hop jam or a guitar solo when you're playing at cocktail-lounge volume. I don't know how many gigs I've been at where we turned all the volume way down, the drummer was playing with brushes, and I practically whispered out the vocals, and there was still some old lady coming up to the band with her hands over her ears asking, no, demanding that I turn down. To the point where I just started playing acoustic guitars at those gigs to avoid the fuss. A different show altogether.
I played a show with The Moshe Skier Band at The Note (Chicago) last Lag Beomer, opening for Piamenta. The audience was all young and middle aged adults. Piamenta did a similar show the next day in a family-oriented environment. The crowd at The Note went wild for Yosi, demanding an encore. The crowd the next day hated him.
It's nice to be able to play someplace where you can crank up the volume, get some energy flowing, and play to a bunch of appreciative adults who love loud music and understand what we're doing.
And you know, maybe make a little Kiddush Hashem at the same time.