It's not compulsory to be a UKPSF member
There are 2 basic options - full & basic
Full membership supports the UKPSF, gives you insurance (injury & liability) - when playing at UKPSF sites/events
Basic membership excludes the insurance and all your money goes to supporting the UKPSF
It's a personal option on going for insurance or not.
The insurance won't pay out if you just trip over a free stump in the woods, its for real injuries and loss if earnings. This is worthless to me as I'm a ling serving civil servant and have probably the best cover via my job/pension plan, if I can't work then I still get full pay for 6 months, then half pay. Not everyone has anything his good. So you should consider going for the insurance
But read the policy small print available from the UKPSF - if you're self employed etc then you cannot afford a short time off work. There are minimum time limits on the insurance (it won't pay because you were too sore to work for one day, or are in a job where you cannot turn up with a bruise on your neck). So you may need other insurance
What the UKPSF does for you whether you are a member or not is support sites, represent paintball with the home office regarding legal situations, and media when there is a news event. If you're interested in paintball being a sport, getting televised and being in the Olympics then being a member is essential to you
Read up here:
http://ukpsf.com/