I would say that any task that made me question the strength of the folding knife or how cofortable the handle is. Such as exerting a lot of pressure on it (need a good roundish handle for that and the lock better be tough), a certain amount of torquing (say enlarging a hole in an enclosure for an electrician to run conduit) or, God forbid, mild prying.What do you consider a call for fixed blades ?
I could add anything that is so messy that the required cleaning afterwards might not be so easy on a folding item.
I think this is the main reason for sure!...., and maybe people are just carrying what the mood strikes not worrying about blade lengths....
Thanks all for all the great answers.
Mikel