Depends on your local situation.
In Australia we need to have a registered/plated vehicle to ride it in public areas, even if it is 100% off-road, as the public bushland is still considered public roads.
Part of this registration process also provides accident insurance, ambulance cover, etc. so it's not only a legal requirement but also an insurance policy/safety-net to do so over here in the event that you crash and get injured.
One other benefit. It helps the resale of older dirt bikes. I sold my 2005 Husky TE510 a few years ago and several people were looking at it only because it was a street legal motorcycle. The did not plan to ride it offroad.
Gettin' too old to be riding without insurance, if I fuck up bad I'll need someone to put this Humpty Dumpty back together again
so anybody really rides 2 stroke on street?
I think it increases the resale value when you have a clean plated title to go with the bike.