I was considering strategies to help my armored infantry style Ork army get into assault range and was looking at Lootas as a long range direct fire support option. I see Mekboyz may replace 1-3 Lootas, but what is the value of replacing Lootas with Mekboyz? The weapon options for a Mekboy do not offer any advantage over the Lootas deffgun that I can see, so why would someone field Mekboyz with Lootas?
Update 8 October 2012 When I asked this question, 40K was in 5th edition, and the accepted answer remains quite valid for 5th ed. Now that we're in 6th edition, a Battlewagon full of Lootaz may fire their heavy Deffguns as snap shots when their rolling fortress moves. Having a Mekboy along for the ride has some tactical value for immobilized/weapon destroyed repairs on the Battlewagon. I do not want to start a discussion, simply point out I find there is now some tactical value for choosing a Mekboy with a mob of Lootaz in 6th edition.