depending on the price on your dates....it might. But you can wait until you actually purchase tickets and sign up at the same time. When you search Spirit they will show you both prices if available. I am flying them in June with 2 families, 6 people total. The price one way was 38.09 for club members and 78.09 without. That's $40 per ticket x 6 people, so it was a no-brainer to pay the $60 membership fee. The bags were then lower as well as seats. I just selected the club member price, filled out the passenger information as usual and it prompted me to join, adding the fee to my ticket purchase.
Now, I've only just joined and have only gotten 1 email so far advising me of an upcoming membership sale, and it did not cover my return date. Still holding out on that one. No $9 sale yet, but I know they do exist but sell very quickly. I can't say I will ever use it past this one trip......but it was worth the overall savings to spend the money this time. It will auto-renew, so if you don't want to remain a member you do have to remember to cancel it.
regards