I don't think a martial controller has to be any more silly than a martial leader.
Here's a guy that is inspiring his allies and creating positive effects across a battlefield by being a really impressive guy in armor with a big voice and tactical knowledge.
A controller is just the inverse of that in many ways. He would create negative effects on a large scale across a battlefield for use against foes, rather than positive effects on a large scale across a battlefield for use on allies. So instead of being a really inspiring (almost comically inspiring - "feather me, yon oaf"??!) person, he could be a really intimidating person.
A dervish or berserker with reach weapons may not affect a whole battlefield, but depending on your spell descriptions for 4e, a fireball rarely covers a whole battlefield, either.
It's AoE for Area of Effect, not Area of Everywhere. I think the "reach" of many AoE wizard spells is being overstated or overestimated here. I doubt AoEs would be significantly greater in radius from 3e to 4e.
Also, combat is kinetic in 4e... it's running battles and terrain hazards and constant repositioning instead of "the wizard stands in the back" - because if the wizard plants himself in anyone spot with plans to stay there for the whole encounter, he'll be in real trouble when the "back" suddenly becomes the "middle". So it's entirely possible for such a character to be making running swipes and dives and tumbles, and all sorts of dramatic movements across space, mowing down foes in the process. The Tasmanian Devil with blades. This could easily replicate "line of effect" style attacks like lightning bolts. The shapes of the AoEs might change, but these things always have varied by class and ability or spell selection.
And if said dervish or berserker takes down a half a dozen kobolds in as many seconds with a single maneuver, it doesn't seem silly that the other three dozen kobolds across the field of battle would be crapping in their already soiled rags. The potential for AoEs is there, and the potential for negative modifiers and influences across an entire battlefield are there.
As for "filling in the grid", perhaps that's the wrong term to use. I don't feel the grid should ever be considered complete. Unlike the "dragon grid" clearly we like our character classes and we don't want them to stop at one of each. But we would like to see one of each, at least early on. We already have two martial strikers to begin with. Eventually, there will be more. But to clarify: to start, having a class for each role within a power source, acknowledging that there will always be more classes in some role/power source combinations.
I think a lot of it comes down to thinking with old paradigms. People are still (and will probably remain) stuck in "static combat mode" mentality or "dipping in other classes weakens your primary role" mentality or "the fighter wacks on one opponent with a sword until he falls and then moves on" mentality, or any of the other tropes of previous editions that they state they are getting rid of.