The importance of lore is directly proportional to the importance of story, if you are going to bother making a detailed story the least you can do is not contradict it later.
Post here series that have insane amounts of lore that is actually interesting
Carter Garcia
Brandon Jackson
This.
People who say otherwise are retarded; part of the reason I hated the recent (ala 2006) TES games was due to lore-breaking.
Michael Green
A point user, just one. There are multiple reasons why lore is important. Another is the reason you gave. Give anyless care to lore is no different than allowing a child write whatever they want, however they want as canon.