You don't necessarily need it, but it is much easier on your transmission. If you are offroading and in a situation where you need 4wd (which is when i specified to use 4lo - "read my post") you are not going to get anywhere near 40mph. You likely won't see 10mph in that situation.
I have years of heavy off roading experience. I'm not just pulling this out of nowhere.
Man, I do not understand where the mentality of "I've read the manual and have the ultimate knowledge - get it right, loser!" comes from. I could understand saying "hey, not sure if you have a reason for recommending something different but so you know the manual says don't use 4lo over 35mph. Would be interested to hear your reasoning.", but "Read the manual."... Come on man, have a discussion. We're all here because none of us are the foremost authority and there's always someone with greater experience. Let's talk like adults and learn from one another.
your statement of leaving it in 4lo comes off as sounding like someone that has "maybe" been off road, barely. not someone with years of "heavy" off roading, you should know well it completely depends on the terrain and circumstance. Besides I don't think there going to be attempting to crawl rocks on there first trip to a off road park with a stock rig, it will most likely be a learning experience which will be where you get to "find out" if you should use 2wd/4wd/4lo given the particular circumstance.
I would never tell someone ya just use 4lo all the time off road, no.......... because it completely depends on if it is wet, dry, ice, snow, rocks, sand, mud, small hill, big hill, giant hill, downhill etc.