No, we do not agree. Everyone who has "answered" this question so far has been, simply, wrong.
Well, let me amend that: it's true that the option is no longer in the "customize toolbar." But use your brains for two seconds and think for minute: if you were going to make Mail even more efficient, where would you put the Contacts button so that it would only appear when it was actually needed, rather than taking up space all the time?
That's right! When you create a new message!
Oh look!
And so once again we find that something people believe to be a "flaw" in Yosemite is, in fact, a failure of user imagination. And before someone asks, yes it also appears in the CC or BCC field. It's almost as if someone at Apple actually thinks about this stuff and how to make it better or something!