Security, secruity, security!
And a better Google ranking since they lower a page that doesn’t support SSL.
Every login on your page is unencrypted and usernames and passwords is submitted in plain text. An attacker can easily read it, copy it, use it elswhere. This is simply dangerous. Plus that many users use the same combination of usernames and passwords for a lot of services at the same time. Hey, the masterkey.
In today’s times I’d not even spend one millisecond in thinking about of using SSL or not. Use it. Protect your visitors, customers and yourself. What happens if an attacker is able to grab your admin credentials?
There’s no disadvantage. Only for the bad guys.