I still stand by my original opinion that it is more misleading to have it in the dictionary than not.
No one, anywhere, really thinks that the misspelling is correct. Regardless of which reason appeals to you most -- traditional historical convention, etymology, current majority opinion, or (my preference) all three combined -- it is absolutely undisputed that 'supersede' is the accepted standard spelling.
Including the misspelling would only be confusing, especially for learners and others who use LEO as their only source. Since LEO does include many true variants under the '(also:)' marking, like 'web site (also: website)' (or vice versa, can't remember), including a misspelling under the exact same format would make it look as if both forms were equally acceptable, which is simply not true.
As for people who find it in a newspaper, what did people do in such a case in the days before dictionaries started including common misspellings? They looked it up under another plausible spelling, eventually found the right one, and quickly realized (duh!) that someone had made a mistake that they would be wise not to imitate. In fact, people are still capable of this process of logical deduction today. It's not really very hard at all, unless we as a society train people out of the habit of thinking for themselves.
Why do such glaring errors appear more frequently today, even in the most reputable newspapers? Believe me, only 15 or 20 years ago every copy editor worthy of the title would have died of shame rather than let 'supercede' appear in print. But nowadays, more publishers are simply failing to hire copy editors (in the sense of people actually qualified to edit and proofread text, not just people who do page layout). And yes, more dictionaries and spell-check programs are failing to mark misspellings as wrong. Do we want to contribute to this trend, or help resist it?
Including mistakes as supposed variants only makes spelling more uncertain, not more logical. English spelling relies only to a limited extent on internal logic such as phonetics anyway. It would not be either possible or desirable to attempt to 'rationalize' it. (Du we wahnt tu spel evrithing funetikly? Shurli naht). The primary determining factor of correct English spelling is tradition, pure and simple. Some spellings -- especially, for instance, compound words, or new coinages -- change in the early stages of a word's entry into the language. That process is normal, and it's only reasonable to include true variants that have roughly equal acceptance at any given period in time. But this word has been in the language for centuries, and its preferred spelling has already been established for hundreds of years, even if the misspelling persisted alongside it. Why try to reinvent the wheel at this late date?