This is related to the post concerning trademark bullying. The purpose of trademarks is to prevent consumers from being confused as to the source of origin of particular goods/services. So when I buy a Pepsi cola I know just by looking at the name/design what to expect. Trademarks also help brand owners to protect the goodwill associated with their business. If you have a trademark and you allow others to use the same or a similar mark without objection, you can lose control of this goodwill. A good example is a client of mine who has an online store. A person on Youtube used their trademark to create a channel to post videos that are in my client's area of business. Now if they started to post low quality or offensive videos, people could very well assume these videos are coming from my client and decide not to purchase goods from them as a result. If enough time goes by, and a bunch of different people start using my client's trademark, the mark could lose all significance as a trademark because people will cease to think of the trademark as associated with one particular good or service. That is why it is critical that you vigilantly monitor your trademarks or hire a lawyer to do this, so that you can make sure your mark is not lost.