It is a D4a that is less useful than a D4. Smaller cargo area. Side door instead of tailgate. Narrower cargo opening. Really, the only plus is the larger tires. Otherwise, it is just a newer Discovery. If it did not have a Defender badge, that is what everyone would think.
It's also 1000lb lighter. 1000lb is not a small difference. You should see the amount of work it takes to drop a few pounds off of structural components. This isn't a small deal.
If you are one of those people that thinks a D4 is great off road, you are lacking in experience.
I regularly trip around the PNW, have driven the entire road system in Alaska over the course of 2mo, been to Moab five times, Death valley 4 times, SW CO three times, spent 6mo in SE Asia, toured NZ, etc etc etc.
And I have yet to be anywhere where my LR3 would hold me back. Wanna see pics of a bone-stock LR3 on Poison Spider? Oh wait, that's an "easy" trail. lol.
So I counter: If you're one of those people who thinks they need a Wrangler Rubicon to travel - you're either in an extreme scenario which doesn't apply to almost anyone on this board, or much more likely you're just compromising like crazy to have extra capability you'll never use. During my 2014-2015 trip which spanned 50k miles across North America I was in a less capable truck (Xterra on 33"s, lift, e-locker, etc), and it was total overkill.
I recently 'converted' a buddy who had this same mindset. He had a lifted 4runner on 33s, lights, bumpers, winch, etc etc etc. His truck caught fire while he was having his rear bumper welded on, and he ended up buying a Grand Cherokee Overland. Guess what? Both him and especially his wife are WAY happier being in a truck which is quieter and more comfortable. And he can still hit 90% of the trails he could before (yes he's aware of the limitations). Not everyone needs the most hardcore solution.