I long ago left Messenger for Mac behind when I discovered Adium but there are occasions for one reason or another why I resort to it.
That doesn't mean I want it installed on my machine however and one way round this problem is to use Windows Live Messenger widget. Windows Live Messenger widget is a very stripped down and basic version of the popular messaging client which is limited but most importantly, allows you to chat.
The interface isn't exactly great and you don't have any audio, emoticon or webcam options but if you don't need any of these, it's fine. Strangely, the widget actually allows you to try and sign in to a variety of accounts including yahoo.com and even gmail.com. However, the gmail.com sign-in definitely doesn't work and neither I suspect does the yahoo.com so I'm not sure why they're there.
When you sign in, the interface is a bit of a muddle and it's not easy to see who is and isn't online. However, with a little navigation using the side menu, you should be able to find your way around. Sometimes it displays a "Page not found" message which further adds to the irritation however.
Windows Live Messenger widget is generally a poor attempt at providing Messenger for Mac in a widget but if you really have no other way of accessing Messenger, it's worth a try.