Use of slang matters, primarily because it is a contextual way of speaking, and may only be understood/make you understandable by a small group. The use of commonly accepted "official" terms or language, on the other hand, increases the chance that communication will be more readily understood. And as harka aritulated so well above, since the purpose of speech is to make ourselves understood, limiting slang can be beneficial.

Let me also say, however, that from a sociologial point of view, communicating in the "accepted" jargon/slang of a certain community or culture can increase trust and credibility within that community. It can create socially important bonds.

In my opinion, what it comes down to is tailoring speech to your intended audience.

 Slang, along with grammar, relatability, and all aspects verbal and nonverbal communication DO matter. I like to think of it as a matter of credibility rather than immitation.

According to Arguments and Arguing: The Products and Process of Human Decision Making by Thomas Hollihan et al, selling your argument (which we make all the time in the form of claims) is a matter of tact. Hollihan et al likened selling an argument to selling a car. If a car salesperson is making a transaction with a couple, he is going to find out what they are looking for. Chances are (hopefully), the salesperson will not hastily recommend a sports car, saying that the husband is going to get all the ladies! That would lose the salesperson some serious credibility, as the couple may doubt the salesperson has their best interests in mind. Hollihan et al tie this into TAILORING your arguments to fit your AUDIENCE. The salesperson will most likely ask them what they will use the car for, who will use it primarily, and the like. He can then give reasons based on their needs.

Similarly, your listener might find your particular slang disagreeable, predisposing them to reject your claims on emotional grounds (i.e., create a bias). If I use slang with an elder, that elder will hear the 'bad' words I used and automatically label me a negative influence, nevermind not understanding what I said. If you say 'bless you' to an athiest after he sneezes, they are going to be on their guard. You've lost some of their credibility (or at least trust). If you shout 'Jesus Christ' in front of a Christian, they may look down on you or at least become critical of any suggestions you give them. You've lost some of their credibility. (or trust again). If a Californian visits the East Coast and says 'no way, dude' to a person exposed to the 'surfer dude' stereotype who has never met a Californian, they run the risk of being labeled unintelligent. Nobody wants to listen to such a person. The Californian has lost credibility. If you quote Shakespeare to a group of gangsters, they may perceive you as snobby, dorky, or corny. You've lost some of their credibility (and probably some teeth). The list goes on, and on, and on...

In short, the vocabulary you use determines how your audience will perceive you. It is crucial to create a positive image through the use of appropriate situation-specific language if you hope to be influencial to those around you.

 Ummmm. Be that for not is together, then the frown is for the upside down, and furthermore, that this aint all water for isn't it nice?
Uhhhh huh, what, right ,huh

 It matters if no one understands you.

 Slang is part of communication in some registers.

 Yes it sets a pictorial tone to communications.

 The use of slang matters because some people can't understand slang words..

 I think using slang does matter when you get to the point where people have difficulty understanding you.

 Using slang matters to an extent. If most people were to use slang as they did in the 60 & 70's it would be ok, but today we are at the point where not only do people use slang, they don't enunciate. They dont use proper grammar, they dont make eye contact. Maybe I'm just getting old, but things in slang these days just dont make sense to me.

 It does. For better communication and comprehension, one should avoid using slangs so as to avoid misunderstandings as well.

 The object of communication is to convey to another person your thoughts or feelings. How succinctly you do this depends on your correct understanding and use of language. Slang is not the only problem. People use words that convey a similar meaning instead of the correct word. Example: You don’t hate your job you detest it.

Hate, detest, loathe, abhor, dislike, despise and abominate all have similar but different meanings.