Good and Evil

[Hatshepsut]

...but emotion must be just as rational as any other process that takes place in human brains. There's no essential difference between the tissues that generate emotion in the brain and those that generate "rational" thoughts...

The areas of the brain in which people reason fallaciously are the same regions of the brain involved in valid reasoning.  That does not mean that fallacious reasoning is just as reasonable as valid reasoning.  But if you were right, then they would have to be equally reasonable...

I've noticed the word shifted from "rational" to "reasonable" albeit as vaguely defined as both these terms are, it doesn't matter. I haven't denied the existence of rules for formal and informal logic, or that a verbal argument can be evaluated for validity and soundness according to these rules. I'll take it that's what you mean by "reasonable." I've merely asserted that emotion and rational thought are roughly equivalent in their overall power to take reality into account.

The only precision rules in logic are those that parallel set theory in mathematics, such as the syllogism which is an expression of set intersection or containment. Violations in formal logic, such as the fallacy of composition, all trace back to faulty syllogism in one way or another. Formal logic is quite powerful but comes at a cost of having to make simplifying assumptions about reality. Categories are treated as if exactly defined and kept few in number; every statement is either true or false so as to use the law of the excluded middle. Unfortunately, many situations in the real world are difficult to approach this way. For instance, when implementing ethics through law, decisions result from a process of personal influence and deal-cutting that largely ignores reasoned argumentation except when writing it up afterward. Almost all legal reasoning is post hoc, designed to justify what has already been decided.

Emotion allows one to rapidly assess the significance of new information, especially if that information is incomplete or contains contradictory elements, without making the categorical assumptions logic requires. Most of the time, instinctive thinking is the more useful to us in the real world, where we don't have time to analyze everything and test every conclusion. The downside is that conclusions reached by intuition or emotion are less reliable than those that can be established by formal reasoning.

Many (informal) fallacies relate to emotion, including the hint of ad hominem I detected in your post at the word "ridiculous." What better way to win than to impeach your opponent's basic intelligence before presenting your own material?  

...So its okay to abuse animals, keep them in conditions worse than slavery. ...there are many negative side-effects of keeping animals in conditions of abject misery.

I'm not sure how the PETA agenda entered this thread, but it's a good example of how reason doesn't decide issues. Those who feel the ethical boundary of "personhood," or perhaps "sentience," should extend to farm animals reach their position early on without any formal reasoning, simply by looking into the eyes of their porcine friends and letting the gut tell them how it is. Later, they'll use logic to aid construction of a public debating position so they don't look totally foolish. Yet a law banning factory farming won't pass until vegans have enough political support, most of which they'll have to garner by emotional appeals to power brokers.