If you like to have a hand in improving the game then yes. If you are looking for a free demo then no. Everything in the beta will be erased once the beta is over. So at the end of the day it is up to you if you want to play it or not. Personally I enjoyed playing the Cata beta. I got to report a lot of bugs and I feel as though I helped a bit with solving some issues. Whether I did help or not is a different story but I like to think I did.
You're looking at the beta experience all wrong, OP. It's not about getting a sneak peak at the new content, it's about testing the new content and helping Blizzard ensure that the product is solid. Your responsibilities include making posts on the beta forums when you notice a bug or if you feel like a particular class mechanic isn't functioning like it should. What you test is really up to you. When I was in the Cata beta, I leveled up a new goblin priest from 1 to about 55 or so. Then, because I really didn't feel up to leveling through all of BC and Northrend content, I created a premade priest to start testing out the new 80+ content. With every zone I did, there was a thread on the beta forums asking specific information. What level you were when you started and finished, if there were any specific bugs you noticed, what you thought of the quests/general experience, etc. I don't think I posted feedback for every zone I did, but I posted feedback for a lot of them. If you want a good chance at being invited for another beta later, make sure to hold up your end and either post feedback in the forums, or do it in game.
One thing that annoyed the crap out of me last time, was the amount of people who would go to the beta forums and whine about how the zone they just did wasn't finished and how they didn't get any quest rewards, and blah blah blah. They were entitled jerks, and viewed the beta as their own personal advanced preview. That's not what a beta is about.