Certainly an interesting discussion.
I could see some point in adding another level. (but then say you did add a 6 (and maybe 7 as the article mentioned) where do you stop, say for example all conditions were exactly perfect...and you got an extreme storm, well beyond those, and the likes of which has never been seen? Do you then add an 8 for such a beast? (something that would likely never happen again))
I think the biggest thing would be as others have mentioned...the public would start going "This one is only a Cat-4" I'm not going to worry about anything less than a 5 or 6.
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@Lanny Dean:
I think in the case of Dorian you absolutely have to take into account that it parked there for a full day.
I know Tornadoes and the EF scale can't be directly compared to a hurricane(since its a damage scale, not simply a wind scale), but say you got a long lasting one not exceeding "EF4-level wind", and it moved at very minimal speed across a town, there's going to a whole lot less left of the place than if it whips through. Maybe even bad enough that it'd end up rated an EF5.