This is not something I push. I encourage, and praise and run an environment that encourages using words, however if they're not quite there yet, I don't rush it. I have a 24 month old who said very few words up until last week when she suddenly exploded into over 100 words. If they can, they will, and most will eventually get there on their own. My own son, had pronunciation problems and was seen by Tyke Talk at age 3. Now at age 4 he is good to go and understood by everyone in time for school. My other little guy is 17 months and has very few words "see" "shoe" "mama" "dada" "more" and he makes counting noises that turn me into mush...I believe he will follow in his brother's footsteps language wise, and that's ok. He'll get there.
I would encourage it more after they turn 2.