Will you, as an entrepreneur, accept whatever your client demands? Will you also agree with your investors' all terms and conditions? Just because you need investors or you need clients (and not vice versa generally) you should not be accepting everything. Does that mean you should argue or fight for your point? No you shouldn't do that. There comes your negotiation skills into play.
Negotiating is a skill because it should be a win-win situation for both the parties. You've to achieve that point of balance without hurting anyone and letting everyone feel like a winner. You should be able to start and be a part of healthy discussion. You should have multiple solutions for win-win situation. If you feel that the person in front is strictly NO about a certain approach propose the next one. Sometimes you might be in exactly opposite role, where the party you are dealing with is proposing various approaches to you. Before such discussions you should have spent enough time, thought, possible approaches as well as their pros and cons. That gives you capability to accept or reject something while the discussion is going on. So even though it appears that your decision is on-the-fly, it is instead very well thought of in advance. You can of course ask for time to think and decide upon a certain approach, when a path totally out of the way is suggested. But remember you cannot always do so! You might lose a deal just because you took time to take decision. In-between period might give your investor or client some party other than you. So even in case you are borrowing time to decide it should not be prolonged.
Having good communication skill does not necessarily imply you've good negotiating skills. For that you should be a good listener as well. Be open-minded and think logically. You might have to deal with many naysayers, be sure you don't get upset and apply your ability to convince at its best.
I always wonder whether ladies are good at negotiating since they are already so used to bargaining! I've still not come to any conclusion. What is your say?