There is no official statement why *Normandy* is not used for strikes, but here it is probably related to the fact that strikes by starship guns tend to level the entire area. Quite often, enemy is located in area that you want to enter or otherwise not to destroy. That is first part, related to firepower. Second is related to the nature of *Normandy.* *Normandy* is a stealth vessel, a submarine in space if you will. It is not a strike craft. While *Normandy* can hide itself in space, for a limited amount of time, it can be seen on surface where anti-starship guns can target it. Furthermore, since the ME fields needs to be generated to allow the ship to fly in gravity field, it's shields are weak. In one of the ME books (forgot which), a frigate was destroyed by shoulder launched anti-ship missile launcher in atmosphere. Often, *Normandy* operates in areas that enemy is heavily entrenched in and as a result, trying to approach these areas for high-precision attack is far more risky than just taking on site on foot. For example, on Feros trying to attack the Geth drop ship would have every geth gun target *Normandy*, which is not build for extended combat. Talking with Anderson and Presley, they will tell you that *Normandy* is ideal for stealthy insertions rather than combat. In other words, being a "bus". It has teeth, but it should only bare those as a last resort. It's primary method of dealing with enemies is to hide and run, not stand and fight. This is best seen in the opening of *Mass Effect 2*. Moment *Normandy* is noticed by the Collectors and it fails to hide and run, it suffers catastrophic damage.