Does my Crossbite Need to Be Corrected? (photo)

From the picture you posted , it looks like you are an adult, in which case I would agree with the recommendation of your orthodontists. In children it is very easy to expand the palate and alleviate a cross-bite, since the palatal suture ( bone)  has not fused. In an adult its a difficult and usually unsuccessful endeavor to expand the palate. You would need surgical assistance to get a stable result and most patients and orthodontist dont think its worth it. If your bite is comfortable, you should be fine to leave the cross-bite in the back.

Some studies out there have shown an association between posterior crossbites and jaw joint problems, but these studies were examining young children and adolescents. 

In asymptomatic middle aged adults (which I am guessing you are) where a crossbite has been present for a long time without causing problems, it is often a different story. Your orthodontist has taken a good look at the health of your jaws and decided that they are in good condition despite a long standing posterior crossbite.

Crossbites caused by a narrow upper jaw often aren't able to be corrected without some sort of minor surgical procedure. Your orthodontist likely doesn't want to put you through that if he/she thinks your jaw joints are not at risk.