As an inspector, I welcome rainy days, in particular heavy rains, because after all, how often do you get to turn on a water hose on a house exterior, let it rip continuously for a couple of days, and then view the results. Yes there is more discomfort and safety concerns (slips, trips, falls) for the participant during a wet inspection but the results and finds for the buyer can be most important.
Long periods of rain stress the landscaping, sepetic system, exterior doors and windows, exterior siding and of course the roof. A compelling case can be made to book home inspections whenever you have rainy days.
For a complete discussion of exterior issues that become more apparent during rains, see my post here.