Well, I mean, it’s prescriptive also. :smack:
Two points here –
First, I suspect British usage is more flexible on this point than American usage – that’s a general pattern that holds true in a lot of other areas of grammar as well (e.g., American usage **mandates **flanking commas for nonrestrictive modifiers, while the British tend to leave the decision up to the author).
Also, though, the Cambridge Grammar is descriptive, rather than prescriptive. That’s not in any way a criticism – if you’re trying to describe the way a language works, few things can be less useful than making pronouncements about what you think people should say, rather than what they do say. However, its (admirable, and accurate) discussion of the two usage patterns for “as well as” cannot be taken as entailing that both those patterns are accepted in any particular publishing house’s editorial practice.
As a parallel, consider the old rule requiring the noun serving as the “understood subject” of a gerund to be marked as possessive (“We were surprised by Bob’s showing up early”). Any good descriptive grammar will tell you that most native English-speakers don’t actually do that, and there are quite persuasive arguments that there are different kinds of gerunds, and that the possessive sounds natural with some of them but not others. For years, though, editors applied the rule anyway. I didn’t matter that it was artificial, and unrelated to people’s actual linguistic practices; in an odd kind of way, the fact that you had to learn to do something so artificial made it an even better indicator that the writer had curled up with a style guide and was playing along. It’s like wearing a tie if you’re male – there’s no functional utility to the damn thing at all, but it does indicate that you’ll conform to the rest of the tribe’s idea of decoration.
Alongside descriptive grammars of English, we could have descriptive ethnographic accounts of editing practices. Absent those (er…maybe they do exist, but I haven’t seen one), we’ve got usage guides, which are a very poor second. But many of the American ones do mandate treating “as well as” as something that does not count as a coordinator for purposes of subject/verb agreement.
You are wrong on several points.
First, there is nothing inherently “logical” about the language rules used to determine whether a verb should be singular or plural. They are mere conventions, and none have more logical weight than any other beyond what is actually communicated to other speakers. If as well as is considered to be a perfectly valid marker of coordination (as is the case in English) then the verb is “logically” plural.
Second, though I agree that the English language is muddy, it is no more muddy than it ever has been, and it is no more muddy than any other language. Your statement that English is not “so far gone” implies that you believe that languages can progress or regress, which is blatantly incorrect. English is not descending into some sort of dark barbarism.
Third, I really don’t care how you think others “should” speak, at least not in GQ. This is supposed to be the place for factual answers. I don’t mind people offering prescriptive advice, even here, but to do so while also neglecting to inform themselves on the actual facts is too much. If you drop an subjective opinion while also neglecting to cite objective facts, then I will point that out.
Epiplexed’s note that some prescriptivist usage manuals proscribe against the OP’s question is valid, and well worth pointing out. I see that my own Garner’s Modern American Usage is one such manual. And it might be a good idea to follow that advice, especially if one must work under the standards of an especially rigid publisher. But the original question was not about the AP style guide, or the Chicago manual. It was a general question about English, and it deserved a general answer. And because every house guide contradicts every other one, it is rank foolishness to rely on such guides when answering a general grammatical question.
Generally speaking, an English writer has a choice, and anyone who says differently is simply incorrect.