My friend Lowry Heussler writes:
-ness: derives from Old English. It is a transformative suffix attached to an adjective, which changes the descriptive word to an abstract noun. The meaning attributed to “-ness” (and really, these meanings are all deduced from context) is “quality of” or “state of.”
-itude: exactly the same, but has a Latin cognate “-itudo,” which occurs with regularity throughout the language.
Although wholly uneducated on the subject, I would have to guess that if one wanted to be quite precise and not pretentious about language, a word of Germanic origin should be paired with “-ness,” and those with Classical roots should take “-itude.”
I suspect that if you pulled all the “-ness” monsters apart (I don’t have the original list) it would turn out that most unnecessary suffix-adding results from too many steps in the conversion between parts of speech. Each Latin or Greek word (if memory serves) has a primary iteration, and then morphs into different parts of speech when a suffix is added. If an English speaker has a word in mind and then struggles with sentence structure, he will often jump the wrong way on the conversion. With the word “perfidious” in mind, he needs a noun and grabs for the -ness monster, instead of stripping away the suffix that turned the noun “perfidy” into an adjective in the first place.
Note that the last point involves some subtleties. “Perfidy” can describe a single act, as in Senator Zell Miller’s perfidy in continuing to hold a Senate seat to which he was appointed as a Democrat while actively working for the Republican Party.
That act may or may not characterize its author more fully: Sen. Miller’s attacks are certainly perfidious, but that’s not the same as saying that Sen. Miller is characteristically perfidious.
“Perfidiousness” might properly be used to distinguish the character trait. For example, someone might say, “Benedict Arnold’s perfidy was, perhaps, merely a soon-regretted impulse; he lacked the constitutional perfidiousness that can alone explain Zell Miller’s systematic and prolonged sellout.”