OK, I'll play the Devil's advocate here with a dissenting opinion based upon a few facts...
First of all most of the ladders that come with RV's, especially C's are very lightweight. Have yet to see one that would make me comfortable (I weigh just under 200#).
Having a permanent ladder is an invitation to bad guys to access your roof. Coaches can be burglarized by skinny burglars {or ones using a kid}, to drop down one of your rooftop hatches. If you store anything up there a fixed ladder is an invitation for thieves to help themselves.
I do have a 12' folding/extendable ladder at home which I use for routine inspections and subsequent maintenance. It is very sturdy and extends well above the roof line to make getting on or off much safer. Having a fiberglass roof {Thank you Nexus}, minimizes the need to be up there very often and I would never even think about storing anything up there.
Every campground/RV park I have ever visited had lots of Fivers and Class A's and most of them have a nice tall step ladder strapped to the their coach. Can't remember the last time I saw a camp host or ranger station that did not have a ladder or two as they use them all the time for various maintenance chores. I can't imagine it would be too tough to find a neighbor, host or ranger with a ladder you could borrow if you had some unforeseen emergency that required your presence up top.
Watched a guy in a Class A use his ladder once to aggressively trim the trees in his site from the roof. It worked pretty well right up to the point where he was thrown out of the CG in perpetuity for doing so... :S.
As far as resale it is a non issue just like slides, levelers etc.. Some folks can't live without them others just don't see the need.
As always.... Opinions and YMMV.
:C