Why is logical consistency so important in ethics? Superficially, logical consistency always seems important for its own sake, but if we dig a little deeper, we're not so sure.
Being fundamentally subjective — the foundation of any person's ethical system rests on her own subjective preferences and her understanding and evaluation of other people's preferences — it's not a tremendously difficult project to make any ethical system logically consistent. Since each different event (action, outcome or state of affairs) is in objectively different from every other event, there's always some objective difference to attach an ethical distinction to. It might be silly, but it's not logically inconsistent, for example, to make ethical distinctions on the basis of gender, religion, national origin, race, hair color, height or even handedness. Since any ethical system — except one that has two distinct evaluations of the exact same event — can be made logically consistent, the consistency criterion seems vacuous.
(Related to the apparent vacuity of the consistency criterion is the fallacy of similarity: one can find many similarities between events and sets of events, and fallaciously argue that the similarity logically entails that the same ethical evaluation be made to both.)
Consistency by itself is substantive only when establishing consistency with some canonical and relatively unambiguous point of reference, such as legal consistency with statute, precedent or a constitution. But when we're talking about the ethical basis of law, consistency doesn't affect the content: any content can be made logically consistent.
But if we dig deeper still, we find the criterion of consistency to be important for a secondary purpose: to establish honesty. For an ethical system to be effective, it must be shared; if you want an ethical system to become socially current, you must convince other people to adopt your beliefs, or at least convince others to permit you to act on them without social consequence. And what might make an ethical system consistent might be incompatible with what's necessary to sell that ethical system to other people.
Once you're in the realm of persuasion, consistency becomes an important check on the honesty of the proponent. If the public justification of an ethical system is internally inconsistent or inconsistent with objective truth, then the proponent is trying to bullshit you (or he may be bullshitting himself). Either way, if you are considering adopting some ethical belief or system, you must look at its consistent formulation to make a rational judgment as to its value.