Google apparently intentionally removed timestamps from the text messaging app, along with colored highlighting to separate users texts; both are in Hangouts though, wow, what a surprise.
They're manipulating their users into using their one-size-fits-all Hangouts app, which not only mashes everyone you contact through any Google service together, such as Google+ people you're following, developers in the Google app store you've chatted with through Gmail chat or Google Talk, and everyone you text through it.
So it's impossible to keep your phone number text conversations separate from your Google+ messages and your Gmail chat messages you do through Hangouts on the desktop. I have personal contacts there along with a bunch of random people half of whom I don't even recognize. Smart, Google. Keep it up.