I won't deny, it's important to us.
Contacts through SLS, or other means are slow and not as interactive. With yahoo (or other chat programs) we can find out very quickly if we're going to be compatible. There's no back and forth of email or messages, no lag time.
To be honest, chatting with someone is often an easier way to get to know them than talking to them in person.
As Julie mentioned, you don't need to have a window to SLS open all the time when you have a 3rd party messaging system. It sits in your taskbar, waiting for messages.
Also important to note - you can set yourself 'invisible' to everyone, or just specific contacts, in the event that you don't want to appear online to some folks.
We created a yahoo ID just for chatting with couples, and it worked out ok. No privacy issues, as we only disclose what we want to disclose over chat.