Does the conclusion follow?  Even if a disposition to be instrumentally rational is indispensable in the way Dreier suggests to having a practical reason, nothing follows about what one has practical reason to do.  Additionally, it’s worth noting that it’s doubtful that Carroll’s original paradox would license a restriction on the scope of theoretical reasons analogous to the Humean restriction on the scope of practical reasons that Dreier thinks the instrumental analogue of Carroll’s paradox supports.  That is, it is not obvious that I have reason to believe Q only if I am disposed come to believe it by applying modus ponens.  Perhaps my merely believing, with reason, both P and P→Q provides me with a reason for believing Q.  Or perhaps the mere fact the available evidence supports Q is enough (Jollimore 2005).