Another kind of illegitimate argument is based on analogy between one area of grammar and another. consider yet another construction where there is variation between nominative and accusative forms of pronouns:
[3] a. They invited me to lunch. b. %They invited my partner and I to lunch.
The <%> symbol is again used to mark the [3b] example as typically used by some speakers of Standard English but not by others, though this time it is not a matter of regional variation. The staus of the construction in [3b] differs from that of It's me, which is undisputably normal in informal use, and from that of !Me and Kim saw her leave, which is unquestionably non-standard. What is different is that examples like [3b] are regularly used by a significant proportion of speakers of Standard English, and not generally thought by ordinary speakers to be non-standard; they pass unnoticed in broadcast speech all the time.
Prescriptists, however, condemn the use illustrated by [3b], insisting that the 'correct' form is They invited my partner andme to lunch. And here again they seek to justify the claim that [3b] is ungrammatical by an implicit analogy, this time with other situations found in English, such as the example seen in [3a]. In [3a] the pronoun functions by itself as direct object of the verb and invariably appears in accusative case. What is different in [3b] is that the direct object of the verb has the form of a coordination, not a single pronoun. Prescriptists commonly take it for granted that this difference is irrelevant to case assignment. They argue that because we have an accusative in [3a] we should also have an accusative in [3b], so the nominative I is ungrammatical.
But why should we simply assume that the grammatical rules for case assignment cannot differentiate between a coordianated and a non-coordinated pronoun? As it happens, there is another place in English grammar where the rules are sensitive to this distinction - for virtually all speakers, not just some of them:
[4] a. I don't know if you are eligible. b. *I don't know if she and you're eligible.
The sequence you are can be reduced to you're in [4a], where you is subject, but not in [4b], where the subject has the form of a coordination of pronouns. This shows us not only that a rule of English could apply differently to pronouns and coordinated pronouns, but that one rule actually does....
For further information, please read section 2.2 Disagreement between descriptist and prescriptist work in Chapter 1 of The Cambridge Grammar of the English Language, published by Cambridge University Press, 2002.