There is, there are, there will be, there was...

Inserting “there”  almost always  weakens a sentence (with one exception, which I will mention below).

The reason is that “there” is a “placeholder” or a “dummy” or even, if we are going to get fancy,   an “existential expletive”, and, as such, adds unnecessary complexity and distraction. There is a man who walks his dog every night means A man walks his dog every night. If you parse the first sentence There is a man who…, you will find it a little difficult to identify the subject. Is it there, which sits in the place the subject usually is, or is it a man ? (It is the latter). 

By recourse to there is you have therefore created a structure in which a non-subject precedes the verb, which complicates the sense because English is essentially a non-pro-drop  language (i.e. unlike Italian, which can say piove, English needs the subject before the verb, even if it is a “dummy” one:  It is raining).

Secondly, in the phrase  There are people who say …., not only do you place there where the subject ought to be, you are also setting yourself up to  introduce a relative pronoun (who) that you do not really need. Try People say or Some people say instead.

I cannot ban it, especially as it is so useful in conversation – but that is also because in conversation, the there is a man who structure gives us time to think of what we want to say.

Exception: “There is/are” is useful, however, when you are counting or quantifying. There are three people in the room is really the best way of expressing that idea. Also, remember, there are no absolute rules on this.