Palestine Today quotes London-based Asharq al-Awsat as saying that even though Fayyad was excluded from consideration in talks between Hamas and Fatah, Abbas is still pushing his PM - to placate the Quartet.
In the words of the article:
Sources pointed out that the reassignment of Fayyad to form a transitional government could reduce the insistence of the Europeans and Americans on the government's commitment to the Quartet's conditions, which provides recognition of the occupation [Israel] and a commitment to signed agreements and to renounce the so-called terrorism.
In other words, Abbas understands that Fayyad is what is giving the PA what legitimacy it has from the US, UN and EU, and also that a government that includes Hamas does not meet the minimal requirements of the Quartet. So keeping Fayyad on until September will give the PA enough political cover to claim to the Quartet that Hamas is really not part of the government despite the much heralded unification agreement that makes it very much part of the decision making process.
Abbas is trying to have his cake and eat it, too. Unfortunately, the West (especially the UN) is so heavily invested in the idea of the establishment of a Palestinian Arab state that he may be right - the Quartet might very well look the other way at the distasteful Hamas involvement until it is too late.