Speed. Falcon makes a good bully ship to support a frigate/destroyer mix.
...Which kind of leads me to thinking that Falcon<->Eagle is simply not where it should be compared. Instead, it is mostly superior alternative to Hammerhead...
HartLord and TaLaR, those are perceptive comments.
Falcon can best be thought of as the functional equivalent of a destroyer in cruiser's clothing.

Against other vanilla cruisers, it is dangerously light-weight and under-featured.
But when compared to vanilla destroyers it is very complementary: the firepower of a destroyer, but with a cruiser's (admittedly a
weak cruiser's) hitpoints and armor it generally outclasses all destroyers except possibly for Sunder (that large energy hardpoint) and Enforcer (same armor as Falcon, and 3 more medium guns) in certain situations. It also has considerably greater flux reserves than vanilla destroyers, giving it additional value in combat.
Of course Falcon is still a
speed king of cruisers, giving it some ability to keep up with friendly destroyers and (refitted with hullmods) occasionally some frigates from time to time - or at least valiantly avoiding falling hopelessly behind, if not actually "keeping up with".

And compared to small ships, it can even be set up as a decent-enough
tank for absorbing enemy fire while your smaller but more numerous partners gang-tackle the enemy in a massed rush.
Falcon can be a surprisingly good hull when primarily used with
smaller, rather than larger, ships. This is almost the only way in which I deploy it. Yeah, it's no contest when comparing Falcon's stats against an Eagle (let alone a Dominator or an Aurora), but as you dudes observed that's not really where we should be looking.