Wasn't the vehicle covering for Hound also a single scene in the cartoon, when they were driving somewhere underwater in the ocean... and then, never seen again?
That rarely used accessory (only able to use it in vehicle mode, and the non-covered vehicle form is what fans recognise as Hound) would have made the price for Hound a lot more than it needed to be as well.

Accessories, yes... but make sure they are worth the inclusion and expense for buyers to get some use out of them, like their weapons, display stand, or a human companion.