Ok...it appears "most" brake fluid now is synthetic, agreed?

If so, and I have the old DOT 3 in my system, must I flush all that fluid out to get consistant synthetic-only fluid?  

Any can anyone tell me of a case where synthetic DOT 3 was NOT adequate for the job of stopping our small vans....

Larry
Ga.

PS. I'd like to stay with what I've already got in the system, synthetic DOT 3 and I have a hard time believing my little daily driver van needs a HP, hi-temp. brake fluid so tell me how I'm wrong?  Any case studies where DOT 3 was not adequate....