You also forget that, as a military vessel, it does not make sense to use on a large scale. I think that although the Alliance fleet makes strives at standardization and uniformity (for the sake of logistics and military operation/organization), they'll use anything they can get their hands on. Therefore, the Falcon is a useful asset, but not a likely expenditure as far as large-scale logistics are concerned. It's also huge - you could pack a few starfighters into the space it takes to house the Falcon. Therefore, it's a really bad idea to put lots of Corellian freighters-turned-gunboats into your flight decks. It's also a lot harder to maintain than any starfighter.
You also have physics and geometric concerns in combat. Size has already be covered as far as storage goes, but the Falcon is a huge target compared to an X-Wing, etc. Deflector shields (especially strong ones) are great, but if you're getting hit at all, there's a good chance you're doing it wrong. Size DOES matter...
Keep in mind that the Falcon probably has a variety of jamming arrays on-board that make it hard to get and keep a lock on - this may explain why it does such a good job at evading enemy fire. To make a game reference, think of playing FS1 and encountering Shivans for the first time: if you're used to using the calculated aim-point and it's not there, you may have a very difficult time getting an accurate shot. Jammers most certainly should play a larger part in SW combat, but they seem to fall by the way-side in most subjects we think about. Regradless, how much better would a standard starfighter do against an enemy with a comparable jamming array as the Falcon?
Finishing with the physics issues (should you choose to consider them) from my last post, even if you have a huge power-to-weight ratio, you have other things to consider. The general force equation (F=MA) is generally only one-dimensional. The Falcon is high-mass, even when unloaded, compared to a starfighter. So, the force generated by the main engines (negating any reactions which affect the total mass of the ship) must be higher than the mass of the ship, especially for a spacecraft. If the ship's engine thrust and mass remain constant, you can therefore easily determine its maximum acceleration. However, this is only in one direction - change the directional vector, and you have a high amount of mass with little force to change the flightpath of the ship. This may also be proportionally true for starfighters, but they were built with that in mind, I feel, and therefore probably handle much better. Starfighters also have much lower rotational inertia values.
Thus, the reason the Falcon does well in combat is the same reason a P-40 or a Polish P.11c does well in combat against a superior adversary: tactics. Whoever is flying the ship is using what it does well to the maximum effect while avoiding puting it in situations that would outright kill the crew. Tactics can let poor equipment or unsuitable equipment overcome a better equipped enemy - isn't that the story of the Rebel Alliance in a nutshell, anyway?
