I was told by a TV news crew that if you keep formatting a P2 card every time you want to start on a new shoot, it will prematurely damage the cards. They said a Panasonic rep told them P2 cards are only good for about 50 or so formattings. They said it was better to just delete each clip.
I always thought you should reformat a P2 card every time you want to start a new project.

I believe they are confused about the numbers. Yes the E series P2 cards do have a limited lifespan but this chart says if used once full daily they should last 5 years 365x5= 1825. I would worry more about SSD drives and other media first. Don't know if deleting individual clips changes the equation or not.

P2 Card FAQs (http://www.p2cardrentals.com/p2FAQs.asp)

I have R series cards since 2009. Never a problem formatting.

Since 1,5 years, frequent use. P2 formatting software doesn't indicate any decrease of lifespan.

I always format P2 cards on the MSU10, PCD1 or other dedicated reader. It's too tedious of having to manually delete each of the folder in the CONTENTS master folder. The MSU10 has a system log and I've formatted over 680 times since 2011 for one of the 12 64 Gb cards that I used. It doesn't make any dent on the life span status display on the P2 Formatter app for Mac or PC. It'll be likely that card will be obsolete before you can use all of its read/write cycles. By then probably Class 4:4:4:4 w/ 400 Mb/s P2 cards will be out and you'll likely get rid of the old 100-200 Mb/s P2 cards by then.

Just deleting clips does not re-set the directory structure so a re-format is always the best option before re-using a card. Mine are now over four years old and not a single problem or dropped frame in that time.