I like seeing distinct life stages because I like seeing my Sim change and grow as they play out their life.
Additionally, a big draw of the game, for me, is seeing my Sims go from cradle to grave. OP does as well, for the same reason-it's sometimes hard to visually differentiate between teens and adults. I'm never opposed to new life states, but if you're asking who cares about teens being the same height as adults, um. At thirteen I reached the same height that I am now as an adult.
Who cares, anyway? Sim Guru Graham said, according to Sims VIP, that this leaves the door open for a preteen life state.