Nope, you don’t have to use the clutch but it’s left in place (the lever) so that you CAN use it if you see fit.
Just personal opinion wise, I’d learn he clutch on a small easy-to-ride bike like a TTR125L. I’ve ridden with the Rekluse and never really cared for them. The clutch just has allowed me to keep traction better in challenging terrain, loft the front end with little effort, and maintain the engine RPMs more effectively. But, I know a lot of folks love them. And granted, if you’re looking at a motocross bike for trail riding, the Rekluse will most likely improve a modern 4-stroke in those situations. I’ve just stepped back to smokers, and prefer the clutching.
Just FYI on the CRF250 you’re looking at...
If you’re looking at the CRF250R, the earlier years (2005-7ish, if I remember correctly) had soft valve faces. Mine was a 2005 and I had to rebuild and put a new head on it. I couldn't tell you the exact hours because I purchased it used, but from my understanding, it was common on the 250/450 around those years and the “fix” was kipplewhite heads/valves. When you start shopping, make sure to ask about a recent rebuild. Signs of the worn valves faces (normally it’s the exhaust valve side) include the exhaust valve being hard to adjust within spec, hard starting, and flameout. But the hard starting is usually the dead giveaway. You’ll kick and kick and it’ll be all you can do to get it fired up.
Good luck! No matter how fun a SxS is...nothing replaces the fun you’ll have on 2-wheels. Even more so when it’s a dirt bike.