User blog comment:Xaero Dumort/Just announced: Mass Effect 3 : Retaliation/@comment-1786741-20121005180140/@comment-4721065-20121007075725

Maybe because a certain segment of their population is tired of simply serving others in this way and want to use their technological knowledge to fight for themselves? In war, people don't always act logically: just think of how the krogan will abandon everyone if they discover the genophage cure dispersal. Some volus engineers probably saw the opportunity and decided it was too good to pass up, regardless of what they could be doing instead.

Also, black ops missions aren't immune to personnel shortages. If a transport carrying a team of salarian engineers for the attack got shot down by geth AA guns, you would have to fill up the shortfall the best way you can and volus mechs could well come in handy here.