Originally posted by: nerp
It's not really good style. A good editor would remove the tired usage and make it more clear.
She's all but sincere should be - she's not sincere. People often write with cliches and colloquisims because they think they sound fancy and it pads their writing. Unfortunatley, it just makes you a bad writer. 🙂
That depends on the type of writing and the audience. Colloquialisms are fine in a society column or for character dialogue in a novel. I'm sure there are other areas where such usage would be helpful in establishing a connection with one's readers as well.
As far as cliches are concerned, the whole reason that those phrases became cliches in the first place is that they provide an effective mental image for a layman. Using the phrase, "monkey see, monkey do" is often more effective than saying, "the subject engaged in the act of mimicry without understanding the underlying rationale behind the actions nor the potential consequences associated with said actions". The former is, of course, inappropriate in a scientific or scholarly work, but for explaining a situation to the majority of laymen the cliche is, in this case, a better choice than the "scientific" explanation.
While colloquialisms and cliches rely upon a shared cultural heritage for their effectiveness (and therefore are inappropriate when communicating to an audience which may not share the same cultural heritage), they remain valid literary techniques when used in moderation.
Back to the OP's initial question, the phrase, "all but" is typically used to denote the condition of being at the very brink of something. For example, a father might warn his disobedient teenager that he is, "all but ready to ground" the child. Such a warning would indicate that, if the child's intransigence persists, a grounding will occur, but that if the child backs down immediately the consequence can be avoided. Saying that someone is, "all but sincere" is to imply that, while the person is attempting to feign sincerity (and may indeed be fooling some people), the sincerity is not genuine.
ZV