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Just to play devil's advocate:
There are certain situations where I'd argue that NULL does equal.
Take score for example - A lead with no score, would have a score of 0, a lead with a blank score is equally 0. While they represent different versions of no score, if I had to prioritize leads based on the score I would treat both as 0 since I've never been given a reason to give the lead a point.
On the other extreme you might have something like "Number of Employees" - In this case NULL does NOT equal 0 since you can't assume that just because you don't know how many people work for a company no one works there.
Thinking about it from a product perspective for a moment, the question I ask myself is - Do I find myself relying on the former or the latter scenario more often, and personally I think that the former does come up more, so in the long run it does save me a small amount of effort (a VERY small amount of effort).
That said - given that Marketo could design different rules for what NULL means per field type - I'm sure they could come up with a much more elegant solution if they tried.