First of all, can we not call it “ghetto black English”? It’s commonly referred to, in linguistics, as African American English, or AAE. The term “ghetto” carries a lot of baggage, and a lot of people who speak AAE grow up in the 'burbs or in rural settings and even in nice parts of cities.
Language is intimately tied to identity. The way you speak tells loads about you. In schools, African American kids are told that they speak a “broken” or “bad” form of English, one that has no grammar, and that the way white people speak is superior. From my point of view, this is a major flaw in our educational system. Ideally, we need to embrace non-standard dialects, show kids that how they speak is regular, has rules just like standard English does, and that the way they speak is not “bad” or “broken.” And from that foundation, kids can learn standard English (which no one denies has an important currency in our culture) from a stance of empowerment. Telling them that the way they speak (and often the way their peers and parents speak) is wrong is almost equivalent in a lot of ways of saying they, as a person, are wrong. This is not a good starting point.
The linguistic choices of African Americans is also complex out of school. So, say an African American kid (or any non-standard speaker for that matter) actually tries to speak as their teachers tell them is necessary. They go home, and they are seen as affected, uppity, an oreo (or a banana or an apple, etc), ashamed of what they came from. Thus, a lot of kids have to be bidialectal if they want to get a good job and remain part of their community.
There are two types of linguistic prestige. There is overt prestige, where a linguistic variable or dialect has the kind of prestige we are concerned about in schools. These are the features used by the ruling classes, and are often deemed necessary to do well in school or in a job. Then there’s covert prestige. This prestige is often afforded to those variables or dialects spoken by lower classes and people of color. These are the things you need to be part of the group. AAE has a lot of covert prestige, even among people who are not African American.
You also ask about comprehension. AAE, when you come down to it, is really not that hard for a standard English speaker to understand with just a little background knowledge. The phonology is a bit different, but that’s just a matter of taking a little extra time to get used to it. The aspectual system of AAE is more of a problem for non-speakers because it is not easily identifiable as different than standard English. But, seriously, standard English is not inherently easier to understand. Since it is learned by everyone in school, it could be seen as easier since it is something we all share on some level, so I see your point there somewhat, but I really think it’s overstating the situation a bit. Language is not just about communicating what you’re saying; much is communicated by how you say it as well.
And, as for writing, a lot of my (college) students speak a form of AAE in the classroom and to their friends, but turn in beautifully written, very standard English assignments.
Here’s some links on AAE: http://www.linguistlist.org/topics/ebonics/
And here’s the Linguistic Society of America’s resolution about the Oakland Ebonics controversy: LINGUIST List 8.57: Ebonics: LSA Resolution
There’s a lot more info out there on the web, but my internet connection is behaving oddly, only letting me go to certain pages.
Also, most linguistic research suggests we emulate the speech of our peers more so than what’s taught in classes or by adults.