Just to clarify - you CAN access the D gates in LAS from the A/B checkpoint. everything is connected post security, except that you need to take a train to the D concourse (no matter which checkpoint you clear, as long as you're not in T2).

It's a long walk and generally not worth it. With the de-hubbing of LAS, the lines have shrunk and the strip is so close that it's definitely worth the trip for a stopppover. I wouldn't make the trip to D unless you have A LOT of time. The time is better spent on a day pass to 24 hour fitness and working out there.