Disadvantage of not upgrading operating system in Apple laptop

Reluctant to undergo the system upgrade in your Apple Laptop? Here are the disadvantages our technical experts want you to know about.

I am having an Apple laptop MacBookPro which I am using since 3-4 years. I came to know that the operating system for this laptop is upgraded by Apple.

Now if I do not upgrade this what will be the disadvantage to me in terms of internet surfing or working online. Is it necessary to upgrade?

Due to the hassles in upgrading I am avoiding it so far .