Hi Jullowyn,
I'll offer a personal view, and then a few professional thoughts about this vexing topic.
For me, the subconscious choice of a singular or plural verb to accompany 'multitude' depends solely on whether I want to emphasize the multitude, as a discrete, separate entity, or if it sounds like--in my inner brain--an adjective meaning 'a large number of' and I wish to emphasize the things, of which there are a multitude. The exception to all of this is common usage. Sometimes I choose to make multitude singular or plural because I'm used to hearing it employed in one way or the other.
Thus, for me, there
are a multitude of reasons to treat multitude as singular, and there
is a multitude of evidence that grammarians follow usage, rather than prescribing a rigid stance.
Mr. Henry Fowler (Modern English Usage, 2nd ed.):
"Nouns of multitude etc.
Such words...may stand either for a single entity or for the individuals who compose it... They are treated as singular or plural at discretion--and sometimes, naturally, without discretion."
That is a sound, logical and sensible BE view. Here is an AE
counterpart, from Bryan A. Garner's A Dictionary of Modern American Usage (Oxford U. Press, 1998).
SYNESIS. In some contexts, meaning--as opposed to the strict requirements of grammar or syntax--governs SUBJECT-VERB AGREEMENT. Henry Sweet, the 19th-century English grammarian, used the term "antigrammatical constructions" for these triumphs of logic over grammar. Modern grammarians call the principle underlying these antigrammatical constructions "synesis".
