Obviously the decision whether or not to testify or not should be made after careful discussions with your attorney. But I would tend to agree with you in that there are a number of cases in which the defendants testimony is beneficial to his or her case. Of course for every good outcome there are probably five bad outcomes so any case needs to be examined carefully before deciding whether or not to testify.
DISCLAIMER: This answer is provided in response to a "hypothetical" question and provided for general, informational purposes and does not create an attorney/client relationship. The information presented is not legal advice and may change based additional information and research. It is recommended that you speak to an attorney to discuss your specific legal issues. This is not intended to constitute legal advertising or solicitation, but rather to educate.
Pleasantness and demeanor are only two of the many factors that are to be taken into account.
As for the second part of your question, it suggests a generalization unsupported by reality, and it categorizes people by status rather than conduct.
Whether a person should testify at trial is based on many factors. I personally start with the premise that IF my client can testify intelligently, and comes across good to a group - appears truthful and forthcoming, and IF the client does not have impeachable prior convictions which might get admitted, then my preference (depending on a boatload of other factors) is that the client testify.
While there is a presumption of innocence, it is a sad fact that despite whatever a potential juror might say about not using the accused's silence against him, when that person becomes a juror then s/he might, in reality, use the silence - even slightly.
Pleasant and well-spoken and genuine are wonderful traits, but the real question is why would I want to put any witness on the stand and the answer is to illicit some information that I can not get elsewhere. I would never put a client on just to show what a nice person he/she is. There would have to be some fact, which I believe is important to being sucessful that I can not get from someone else or through some other means. You always take a risk allowing the defendant to testify and especially when they think they can outwit the DA. I typically know exactly what I want in evidence and a defendant often offers far more information into evidence than I was seeking. The defendant may believe that they are helping themselves, but it is rarely the case. The problem is that you put the defendant in a position where the DA can ask them questions that have no good answer. "I was nervous and that is why I missed the alphabet letters, not because I was drinking." "Oh, so you were nervous about being arrested were you?" Why would you be nervous about being arrested if you were sober/innocent?"
You can see how innocent sounding statements can be used against you.
If you tust your lawyer, heed his/her advice. If you don't trust them, get a lawyer whom you do trust and then heed his or her advice.