I think that the problem of arguing for absolute ethics is just that: that unless you have some deity enforcing this absolute, it doesn't really exist. Or at least, if it exists, it is neither permanent nor persistent with everybody.
Consider this: you may be of the opinion that revenge is an ethical action, and likewise might expect others to act vengefully, and it would seem alright with you, and if someone acts vegefully against you, it would be because of something yuo've done to them. However, others wouldn't share the opinion, and believe that a world where there is no retribution of this kind would be better.
In their own way, they are both right. This is defined by the kind of person you are. Absolutist ethics forget the pragmatic approach that ethics needs and doesn't consider that, while we might be equal as far as rights go (or should be), we have our own set of internal ethics system. Some people's systems just might hold up satisfactorily with themselves, other's not. But the point is that you cannot expect different people to act uniformly, even if there was an imposed, external ethical system, and everyone was aware of it.
Take the example of an extreme situation: the Apocalypse is coming tomorrow and you can only take three people to progress with humanity. Suddenly an ethical system which placed an equal value on the lives of everyone has just created criteria for valuing people's lives; if you intend to preserve humanity, not only would you take 3 brilliant, young adults, but 2 would be women and one would be a man (it is more likely for the species to survive if you can impregnate 2 females at a time. also, should one person die, it is more likely that you could go on with 2 females rather than 2 males).
So, have we suddenly assigned a value to human life. Are women twice as valuable than men? Well, of course not, at least outside of such limit situations. The problem with any absolutist ethical view is that it breaks when you test it against these worst-case scenarios. And if an ethical system cannot be applied uniformly, then it's not really useful, is it? Nor can you define limit situations, as they are limitless (no pun intended) and subjective.
And what happens when you need a person acting irrationally to act on reason? Take for example the maternal instinct.
You cannot expect a mother to place higher priority on anyone that's not her own child. I wouldn't say we should go to the extreme of having her pick between her child and the whole world, but certainly we see a bias that no amount of reason could change. Same thing with the self-preservation instinct.
Such is the reason I belive no only that an absolutist view on ethics is equivocal, but that we could even question the value of subjective ethics and the very existence of an ethical system which could be applied uniformly to even one person. Sure, in some philosophical made-up world it could work. But let's not forget that we live in our beloved Earth, where resources are limited and human irrationality can be limitless. Ethics is an abstraction of that reality too, so I'm sure that a system made by irrational beings will probably break down at some point.