Although I could underlet from my HLT to my mash tun, I just gravity fill my mash tun with the required amount of liquor (my HLT is higher than my mash tun), heat it to strike temp and stir in the grain. Now I admit I am not a LODO brewer, but it makes things easier for me and I always hit my mash temp target.
If I ever decide to build a new brew system, something that has been bouncing around in my head like a BB in a boxcar, I might consider a setup that would allow me to underlet with a pump.