I hate it for so many reasons:
a) the obvious reason, that no-one ever tells men they "can't have it all". They do often have it all, or the good bits of it all at least, and nobody bats an eyelid or suggests they are failing either their employer or their children (or both), a charge which is routinely leveled at working women with children.
b) the less obvious but more pertinent (to me) reason, that in many cases women have to have it all because there's no other sod around to do it. This was prompted by another thread about how single mums work. Well, the short answer is, in the vast majority of cases, because they have to. I know that in a number of cases its not possible and I don't want to minimise this, but there's still this underlying assumption (which pops up on here among other places) that "career women" are doing it because they want to be a feminist, or because they want a nice car or a flash postcode or some other such bollocks. The vast majority of working women have to work and shouldn't be made to feel that they are failing their children because they provide for them.
c) there's often a really reproachful tone when people use this phrase, as if women who work and have kids (and sometimes, whisper it, other things in their lives like hobbies, volunteering and social lives) are being utterly selfish and putting their children too far down the list of priorities. It assumes that any woman who has any priorities at all in her life aside from her children is selfish.
A friend of mine who is otherwise progressive, tough and sensible used this phrase the other day. She is struggling for many reasons at the moment and may have just been having a bad day but it really brought me up short.
The more we use this get out of jail clause, the less we think about the underlying societal reasons why women don't get the support they need to live their lives and bring their children up properly, and the less we campaign for the equality which not only we -- but the whole of society needs.