My thoughts are that for most play bikes a lockup is added cost, added stress, added weight and more shit to go wrong. if i want the clutch to slip i will slip it.
Quality clutch kit, quality oil and regular changes go along way to holding power.
but with that said i dont think old windy john is far off in his estimates if you ride hard. especially in the sand which is notorious for being a parts killer.
i run an 8plate clutch with 2 unknown heavy springs and 4 stock springs. decent pull and plenty of grip. clutch kit is the $40 tusk kit. has about 2 years on it now and still going.