Butter, like most fats, is actually quite resilient to microbes. The problem you are more likely to experience is rancidity. Fat goes rancid by oxidation. Exposure to light, heat, and air cause oxidation and accelerate the process of butter going rancid.
Putting butter in your refrigerator addresses the heat and light, but does nothing for the air. The paper that butter sticks are wrapped in are quite porous. You can see evidence of this as a rind that develops on your butter after a couple of weeks. Butter can also absorb odors in your refrigerator through this wrapper.
A butter crock, as shown in the accepted answer to this question actually serves to protect the butter from air, light, and heat. Butter does not need to be kept below 40 F to stay fresh. The butter crock referenced there will keep butter fresh up to temperatures of 80 F, and likely higher, but the butter will be too soft and slip out at higher temperatures.
A plastic or ceramic butter dish are similar in function to the paper that wraps butter. Ceramic is unlikely to seal at all, so the butter will still be exposed to air. Plastic, while capable of a great seal, will still trap oxygen in the container with the butter, allowing oxidation to occur.
A butter crock really does seem to be the best of all worlds. You get easy access to soft butter, with little risk. Mold will eventually grow on the butter, but this should take weeks at room temperature. If you don't use a stick of butter within two weeks, you're doing it wrong. :-D
Generally speaking, butter spreads have a water component to them. That's what the emulsifier is for, as it keeps the fat and water from separating.
The problem is that you can't fry/cook with it. If you place it in a pan it will separate, and sizzle in a very unpleasant manner. If I remember correctly, it actaully smells quite awful in the process.
As for baking, I think it'll be fine anywhere there needs to be fat as fat. Wherever you need fat as something to hold the structure, you had probably best not use it. Caveat emptor, as I don't bake very much at all.
Let it be aid, however, that margarine as opposed to butter spread, can be used anywhere that butter is used. It isn't as tasty, and has trans-fats. On the other hand, it has less cholesterol.
Best Answer
Well, it seems you answered your own question, though you then expand this question of fat content to "quality."
"Quality" of butter is subjective. It depends a lot on application. Higher fat does not necessarily mean better flavor or better performance (particularly as an ingredients in things like baking). See, for example, this article, which discusses the ranking of butter in a number of applications. Fat content varied from 80% to about 86%, but that wasn't highly correlated with superior results. The tasters' opinions there came down in favor of the lower-fat butter for applications like pound cake (where they claimed more steam from the higher-moisture content added to lift) and buerre blanc (where they claimed the excess fat unbalanced the flavors of the sauce).
But I think this is probably more evidence that American recipes are likely calibrated to assume butters on the lower edge of spectrum, around the legal minimum of 80% butterfat content.
Anyhow, my personal experience is that processing matters a lot more than butterfat content. In particular, I generally taste a strong difference between cultured vs. "sweet cream" butters. And salt content can make a huge difference. But another thing that came up is storage/wrapping, as fat tends to absorb odors and other flavors, as mentioned in the above article and in a Cook's Illustrated review article that also ranked butters in various applications. I find it interesting that this article is available on the Vermont Creamery website, which makes a big point of how it has the "highest butterfat content you can obtain when making butter" of 86%; even they are posting an article whose title is basically that butterfat may not matter as much as other factors.
(By the way, I'd personally disagree with the CI characterization of cultured butters in general as having "artificial, margarine-like" flavors. But everyone has their own preferences, and I think American palates have shifted to expect "sweet cream" butter.)