40K Orks: what is the tactical value of having Mekboys in a mob of Lootas?
Most people don't field the Mekboys.  In fact, in 5 years I've never seen anyone put a Mekboy in a squad of Lootas.

So, you're not alone in not seeing their usefulness.  Technically you could use the Mekboy to build a little duality into the unit.  This makes them slightly more well rounded so as to be able to deal with other types of threats, but that's not what people generally take Lootas for.

Lootas are Orks most consistent and effective long range shooting, adding a Mekboy degrades their points efficiency.  

In response to the edit about 6th

In that case I think you are better off with an attached Big Mek.  That way you can add in a Kustom Force Field to protect the battle wagon.  I'm also not a big fan of just snap shooting them from a moving vehicle.  They'll be susceptible to stun and shaken results on the wagon even if they disembark afterwards.  Going from a 5+ to a 6+ to hit may not seem like much, but is still a 50% reduction in shooting effectiveness.  Why do that if you don't have too?  And finally, why would you need to move a long ranged unit like Lootas in the first place?  A good deployment position should give them range to most of the board.  Put something else in that wagon; something you want to get closer to the enemy.