The following opinions are my own and do not reflect the views or opinions of Microsoft. Microsoft is a registered trademark of Microsoft Corporation in the United States and/or other countries.
There's several angles here to using SSL for retrieving email - the one presented here is specifically for grabbing mail over a web interface rather than through an email client (such as Outlook). In this case, the argument is that because the session isn't encrypted, it can be hijacked by someone intercepting the session traffic and going on to do bad things. While this probably (note the word probably here) wouldn't happen in your home, it's probably more likely to happen where your connection traffic is easily monitored by others, such as an unsecured wi-fi connection.
So is this a good idea? Yes. Does it protect others from reading your email? No - the whole nature of email is that while it travels the web, it's relayed by several servers (usually your ISP's, followed by the recipient's ISP's) and given most messages are unencrypted they are easily read by anyone relaying it. A big step in ensuring your email communications are secure is not sending or receiving sensitive information without encrypting it first.
Does that mean I personally don't use SSL when checking mail? No, I most certainly use it. But I have a different twist on it - I use a mail client (Outlook) to send and receive email, and I use SSL connections to ensure a) my password is encrypted and b) the messages are encrypted while they are synced with my mail client. This is mainly an extra layer of security rather than an exclusive layer in that if I'm at the airport or something and checking my mail then someone sitting nearby monitoring wi-fi can't easily see my passwords and messages. Of course, I add even more on top of that, such as using a cellular connection and bypassing the wi-fi network, or if I am on a wi-fi network, I VPN into a known good network and tunnel all my traffic over it (encrypted of course).
Back to this case with Gmail, yes, it's a good idea to use SSL, especially when over an unsecured connection, but not only to protect your email contents, but more to protect your account (not only can they see your mail, but they can send mail, or grab your contacts, or ...).
