I know this response is coming in way late, but maybe someone else will have the same question about whether to purchase pets or not.
First, as the variety of responses above indicate, I don't think it's an easy question to answer. I'm an older player, geeky, decidedly non-hip, and I love TS3 Pets to death! But then I was also raised on a horse ranch and have always had a managerie of cats and dogs about me. For me, a home without pets (and this goes for my sim homes) is barren. I have all the EPs, including Showtime, and Pets remains my hands down favorite.
If you're the kind of player that enjoys wringing every ounce of juice out of the game Pets has much to add -- interactions, additional daily challenges in caring for your pets; and if you're a horse person there's really quite a whole new world open to you with respect to building ranches, breeding/selling horses, competitions, and many of the daily things associated with farm life. My sims are always multifaceted -- that they're usually gardeners reflects my agrarian upbringing and the addition of horses to the farm only strengthens my overall sense of enjoyment. If these things don't appeal to you or if your gaming enjoyment stems from achieving short term goals, then Pets may not be worth the expense for you.
I hope this tome achieves some measure in responding to your question.
