Most exterior wall cracks are the results of joint movement. The weather can hinder your repair efforts as well - because temperature and relative humidity play important roles with patching materials. As with most of the products you use, materials should be applied within certain temperature and humidity ranges. Apply them outside the recommended ranges and you're asking for trouble. Cool weather and most patching materials do not mix well, especially for those that contain resins, because the resins don't get a chance to coalesce - and therefore the material won't cure properly.
Humidity also is a major concern.
Low is better than high. There may not be any major nicks and gouges on the surface you're going to coat, but there will almost certainly be some small ones. Even the most careful scraping and sanding can still leave spots where the bare wood is much lower than the old paint, or you may accidentally nick or gouge the wood. Either way, these surface imperfections should be fixed if they are in a highly visible location - such as a front porch or entryway. The substrate you'll be coating is one factor in determining the patching material you'll use.