I agree that it does add to the play value of the alt-mode, but it's not a deal-breaker for me. And you don't have to be a Headmaster to have this feature anyway. Look at figures like TR Wheelie, Bumblebee, Chase and Brawn - none of them are Headmasters but they all have functional cockpits for drivers, just like the G1 Diaclone Cars, Dinobots and Insecticons. BT/ALT and Human Alliance figures all have functional driver's seats too, yet none of them are Headmasters. Look at Platinum Edition Omega Supreme. They removed the Headmaster gimmick on that toy but the alt mode isn't any better or worse than Energon Omega Supreme's because of it.

I think that having a functional cockpit and being a Headmaster are two fundamentally unrelated gimmicks. Heck, not all Headmasters even have true pilot compartments - Weirdwolf, Horri-Bull, Squeezeplay, Fangry etc. - they all just have storage compartments for their Nebulans, not cockpits. I'd say that even G1 Nightbeat and Siren don't have true cockpits either since their windows are solid plastic. There's no way you can see Muzzle or Quig sitting in there. The original G1 Optimus Prime toy on the other hand has multiple pilot seats - 2 in the truck cab, 6 on Roller and 3 on Combat Deck! Seeing those little seats in there totally helps support the notion of it being a vehicle.

Star Wars collectors would want a cockpit and pilot with their vehicles because the vehicles are meant to be piloted. The pilots are the characters, not the vehicles. Transformers is different because the robots are the characters. Okay, it's really nice that MP10 comes with Spike and MP Magnus comes with Spike and Daniel etc., but really... it's not the core play value of that alt mode. None of the MP Cars come with drivers and I don't think anyone cares. Look at Binaltech/Alternators - none of them had drivers, but boy are they great toys! Binaltech Asterisk and KISS Play later gave us drivers... but you look at say BT/ALT Lambor vs BTA Sunstreaker. Does Lambor really suffer from not coming with a race queen driver? Is BT Broadblast an inherently better toy because it comes with Lumina and Skids doesn't?

While having a cockpit and/or pilot is nice, I don't find it essential. And it can be done without making them into Headmasters. Just build in compartments for pilots and include small driver figurines - just like Diaclone!

DI-A-CLONNNNNNNNE!