The thing is I consider it not cheap for what you are getting, or are giving up. If Orks had the option to split fire in a manner that their Rokkits could shoot a second vehicle while the Shoota Boyz shot a nearby infantry unit, could Fire on the Run, or there was a better ratio of special weapons to Boyz (1 in 5 Boyz may take a Big Shoota or Rokkit Launcha), I would consider the option.
-For the cost of adding another 10 Boyz+Rokkit to a mob, I can purchase a squad of 2 Warbuggies with Rokkits, effectively tripling the AT firepower, drawing firepower away from the Troops, and providing screening and sacrificial move-blocking that I would otherwise not have.
-For the cost of 20 or so Boyz+2 Rokkits, I can pick up 3 Rokkit Kanz, which also provide a better ratio of anti-tank firepower, a better proliferation of Powerklaws, and require heavy weapons to reliably destroy.
As it stands, Boyz are predominantly anti-infantry; their Rokkits cost too much relative to other options to make semi-reliable, while also taking up a lot more space in the process. One or two medium-sized shoota boyz is good for making cheap objective-holders, secondary assault sweepers, or light sniper duty, and that's about that really.