Like most people here said, to each their own. For me it's more about why you do/don't believe in marriage and what factors affect your desire/willingness to get involved in the ceremony/practice.
Some people simply don't want to be tied down while some value the security of having a sort of "contract" between them and their partner. Both valid.
Some people feel that it's just a slip of paper/certificate that you don't really need while some value it as one of the ultimate forms of expression of love. Again, both valid.
Some people think it's the be-all end-all and that not doing so would mean that you are unfulfilled as a human being. Not-so-valid in my opinion but as long as they're not applying it to anybody else but themselves, you do you. Or I guess, they do them? I don't know, the English language is weird.
I personally don't see it as a must, but would be open to the idea if my partner really wanted it.