This can be a nifty tool to give your users more options.
Gives recipients another way to view the email if their email client does not render it properly
Some email clients do not load some HTML elements (for example, Outlook does not load background images). This provides a way around that for your users.
Makes the email less clean or streamlined, although it is relatively unobtrsuive. We also now have a View As Web page Link token so you can place this wherever you want so you can keep it consistent with your email's formatting.
Most likely not too many of your recipients will use it
Overall, if you can incorporate the View as Webpage link without compromising the asthetics of the email, I would suggest to include it.