[quote=Sir Anthony Kenny]There is no reason why someone who is in doubt about the existence of God should not pray for help and guidance in this topic as in other matters.
If he is asking for any reason (as in motivation) to present to an agnostic atheist to avoid prayer then I would agree in general because there are few reasons of which I know to advise an agnostic not to pray.
Though I can think of some reasons why a person with such a disposition might not want to do so (such as keeping their words and actions as an authentic representation of their stance) or if one was willing to do so I can see why she might not want to do it publically (such as not wanting her position to be misrepresented).
Whether or not the person has a motivation to pray to an unknown god/God is another question. In the scenarios present the people are in a desperate state wanting help. People in these states tend to be more malleable and open to various types of conversions or convincing (both religious and non-religious). I suspect that the more emotionally troubled that one is the more effective that attempts to get the person to pray will be. I also suspect that a person that feels that their life is going pretty well might not be as responsive to attempts to convince her to pray.
Agnostic theists may already have her own motivations for praying without being prompted to do so.