IM: I'd rate passing as even more important than defense, and stamina is actually as important as PM for those guys, I think. As for the secondaries (passing and defending), I think their effect is the same, but on different ratings... passing for your attack, and defending for defense.
CD: passing is useful if you want to play CA, since your CA skill is, amongst other things, based on your defenders passing skill. Furthermore, for an offensive CD, PM and stamina are important as well, but I'd recommend playing a CD normal, thus focusing on defense.
Winger: Winging is the factor that decides on how much the side attack is improved, but his PM is quite important too, they contribute about half as much to the midfield with their PM skill (and stamina counts as well of course) as a normal inner midfield does, so the effect is certainly not negligable, I think. Passing is useful as well on these guys.
Forward: scoring is uber-important, passing helps a little, but not very important, and really expensive on the market atm..
So as for your final question, in my opinion only on wingers is their secondary worth paying for (especially PM/stamina, passing is nice too), all other players I think should be bought mainly with their primary skill in mind, especially for newer teams.
