On the scope of style guides
Contrary to popular belief, a style guide is not a description of the speech of the average person, nor of the majority; it usually concerns a small group. In fact, it does not even describe the language of a group: instead, it is usually meant as an ideal. This ideal may or may not be close to average language; it depends on the guide. All sorts of rules are set out that the author believes would result in better language if followed to a certain degree. What is better language? That depends on the guide, but it is usually a combination of beauty and clarity. The primary goal of a style guide is prescription, not description. This is an indisputable characteristic of the genre. It should also be noted that most style guides concern themselves only with (semi-)formal writing.
I will admit a single point of inconsistency: sometimes a style guide may appear to make an assertion about actual usage and use that as an argument. This could be held against it as a false factual claim if it isn't true. However, such an assertion should not be taken literally, without interpretation; it should be read loosely. A style guide is not a scientific article.
Its prescriptivist nature notwithstanding, a style guide may present factual claims about usage or linguistic arguments to support a certain rule. But for the most part such claims and arguments merely illustrate the rule; they cannot prove it. It is ultimately the authority of the author that carries his claims or collapses under their weight. If we trust this author's taste, experience, and intuition with regard to a certain rule, we will follow it; if not, we will ignore it. The author is free to discourage a construction that 99 % of the people use if he so chooses.
From this it follows that you shouldn't consult a style guide if you want to test whether a certain construction will be acceptable to the average person, unless the guide is written with that particular purpose in mind. That certainly excludes Fowler's. However, if you like Fowler's style and are persuaded by his experience and intuition, and if you want to appeal to a certain smaller circle of language enthusiasts, his guide can be a very interesting source. I believe a good style guide will earn our respect if it leads by example: it should count for something if the guide is well written. That is probably one of the reasons for Fowler's enduring popularity.
Burchfield on the use of pronouns with gerunds
The eminent scholar Burchfield has edited the third version of Fowler's Modern English Usage. This edition is quite different; it is considered much less prescriptive than the previous one (which was edited by Gowers). He has some sort of personal corpus centred on British and American newspapers and appears to describe in some articles, prescribe in others. He is rather descriptive in his article on the possessive with gerund:
As the 20c. draws
to a close the choice of construction
is mostly resolved along the following
(i) The possessive with gerund is
frequently used when the word before
the -ing form is a proper name or personal noun (e.g. Andrew, Reagan, sister,
baby): ... wondering if he should be angry ...
about May's sleeping with him and then
throwing him out, about his grandfather's
having left no message or sign for him but a
field of junked cars—C. Tilghman, 1991
(ii) When the noun is non-personal, is
part of a phrase, or is in the plural the
possessive is normally not used: ... Mrs Thatcher herself is not
averse to this elegant bone being cast before
her long-standing tormentor—Daily Tel., 1987.
(iii) With personal pronouns, where
there is a difference of form, usage is
evenly divided: (possessive) ... 'Is it all right?'
he asked, needing reassurance. 'My coming to
your party?'—B. Rubens, 1987. (non-possessive) ... There would be something so
despicable about him blustering ahead with a
palpably unsound argument—C. Chambers,
(iv) With indefinite pronouns usage is
divided, but the non-possessive form is
now dominant: (possessive) ... Mrs Longo has nothing
against anyone's being Japanese, of course—New Yorker, 1988. (non-possessive) ... should
we not primarily be looking on Aids as a
symptom of something having gone fundamentally wrong with our attitudes to
sexuality?—Daily Tel., 1987.
Further outlook. The possessive with
gerund is on the retreat, but its use with
proper names and personal nouns and
pronouns persists in good writing. When
the personal pronoun stands in the initial position it looks certain that the
possessive form will be preferred for a
long time to come: e.g. His being so capable
was the only pleasant thing about the whole
dreadful day—E. Jolley, 1985 (Aust.); 'My
being here must embarrass you,' she says—New Yorker, 1986. The substitution of Him
and Me would take both sentences into
a lower level of formality.