1) We must have different ways and forms of communication so that we can address different people with different ways and content. Like formal and strong words go together with formal audiences
2) I would say "That is a brilliant idea, but it has its weak points and maybe we should..." or "I see your point but maybe..." so as to state the class major view but also to push my view and have a comparison between the two view, stating the pros and cons and change the class' view and support to the oppsition idea to my idea.
3) One may be using informal tone to talk to some important people for example a speech. Instead of saying "We must help those in financial need so as to raise our standards and economy" the wrong way would be "We got to help them or else our economy drops". The result will be the audience(formal) Will not be satisfied as the words and tone used are informal and do not have a major impact. This may result in the audience not pleased and may have worst outcomes. Informal cases, like talking to friends "Hi, i had not seen you for some time. How have you been?" into "Hello, i had not seen you for 2 weeks. I want to ask if you are fine and well." People will laugh at the formality as in this case it is an informal audience.