I like rokkits on boyz mobs for three main reasons.
First, it enables the mob to fire at transports on the way to the assault. If they immobilise the transport, great - their Nob now auto-hits with his klaw. If they blow it up, even better - now they can assault the squishies inside. If they do nothing, you haven't lost much. It's a similar situation assaulting drop-pod dreadnoughts, too - with a bit of luck you can blow off an arm, immobilise or stun it, reducing the casualties you suffer in the assault.
Second, they're great for defending objectives against pesky transports that you ignored earlier because they'd already unloaded their cargo. A rokkit boy makes a great sacrifice against tank shock by a Rhino or Chimera - the auto-hit really helps their effectiveness.
Third, they cause more casualties against Space Marines than big shootas. Yes, including all the shots you get. Since the main thing my boyz have learned to fear above all is a big unit of counter-charging Space Wolves, a couple of casualties before the fight starts goes a heck of a long way towards coming out on top.