The theory is that it's quicker...which it can be depending on the time of arrival...what they do is spin you through "THE TUNNEL" to 215 and then exit on LVBlvd via the highway..
to avoid it tell them NOT TO TAKE THE TUNNEL, but
go down Tropicana and then cut over...you can
save even a few more minutes if you tell them to cut left on Koval and go the back way, avoid the Strip's busiest intersection (Trop and LVB)...
if you arrive around the time of a big event a t MGM Grand (fight concert, etc), the TUNNEL may actually be the best route to avoid the gridlock around MGM...
Hope that helps!
VE!