Normally, I'm all about adding as much play value to these toys as possible. However, if I buy a Stormtrooper figure, then I would like for the helmet to actually resemble a Stormtrooper's and not some soft, squishy, poorly-painted, semi-gelatinous blob that stretches and warps when you try to place it on the figure's head.
In many cases I would love to army build imperial troops but just can't justify buying these figures when their helmets are so poorly made. The Legacy AT-AT Driver and TAC Stormtrooper are perfect examples of figures that actually took giant steps backwards in detail and accuracy with the addition of removable helmets.
Even when you build large armies of them, they don't look like imposing Imperial troopers. They just look like fanboys from a second-rate Star Wars convention who could only afford to purchase bargain-bin helmets from the local Halloween shop.
I'm not even going to reopen the whole Jangohead can of worms again; but as a general rule of thumb, if the trooper never removed his helmet in the film, then there is no need to release him with a removable helmet as an action figure. I think most fans of Imperial troops would prefer accuracy and crisp, detailed helmet sculpts over superfluous clone heads underneath rubbery, misshapen helmets.