100% needed - part of your safety equipment in my opinion. If you ever need assistance once you will be a fan for life. Not only from the cost but also the convenience. If you break down at sea, even close to shore, you are really stranded unless you have a dinghy and an engine. $300 minimum charge for a visit without a policy vs. $125 p/year for the top plan with Tow Boat US. Choose whatever company is best represented in your boating area. Maybe Sea Tow has more boats/locations in your area. Check that out before you choose to sign up for a plan.
I would also suggest you do a search for an owners club for whatever brand you plan to buy. I own a Bayliner and the Bayliner Owners Club is the single best resource available to me. There are several marine mechanics (who own Bayliners) that reply to questions on the board for example, etc., etc. Others offer information on various systems, information on a new area you may want to cruise for example, etc.
--Matt