I never sign into the old Talk on my gmail account because I find it very very distracting. The presumption of Talk (or other IM clients) is that if I am online, then I am willing to be interrupted to answer your message immediately. That's absolutely not the case --- if I'm working, I'll answer your message later.
The Hangouts (SMS-like) model is much more useful to me --- you can always feel free send me a message, but I will answer it when I see fit. Hangouts gives a hint as to whether I'm likely to read your message soon, but there's no promise of a synchronous response.