The US can use the Pakistani influence, like it did for China, to dissuade Iran from being hostile to US and Israel and ensure the stability of the region as well as securing its interests if the US-Pakistan-Iran trio remove the huge skepticism existing between them. The two countries want to improve their ties because Pakistan wants to counter the influence of India by making good relations with Iran while both have strategic and economic interest in the neighboring Afghanistan having its borders attached to these two.
During the 1990s, however, the things were topsy-turvy as both the countries had different interests vis-a-vis civil war in Afghanistan between the Pakistan-backed Sunni Taliban and the Iran-backed Shiite Northern Alliance after the defeat of USSR. The post 9/11 situation where Pakistan has abandoned its support to the Taliban regime and has become an important ally of US in the war against terrorism has once again provided an opportunity for Iran and Pakistan to develop trust between them (Montazeran ; Mumtaz).
However, it has also increased the Iranian apprehensions against Pakistan that it might take the side of US which has declared Iran as an axis of evil (Zehra). The US, with the help of Pakistan, can co-operate with Iran as it is Iran that could end the agitations in Iraq. Also it is Iran that is staunchly hostile against Israel and lends considerable support to the Lebanese militia Hezbollah.
If Iran plays its role along with the help of US and Pakistan the Middle East region can be free from tensions and fears. Also Afghanistan issue can be handled well if Pakistan, Iran and US co-operate. If the US does not ensure that it wont invade Iran and wont exercise its hegemony over middle east region by pillaging its oil resources there would always remain prospects of US being a target through the worst kind of terrorismthe suicide bomb attacks.
Nasim, Zehra. Pakistan-Iran Relations: Compulsions and Conditions for a Strategic Relationship. Strategic Studies. Spring Number 1 (2003) 08 May 2007.