My opinion, for what it's worth, is that no, it's not essential to have 4wd, but nice if you have it.
A 2wd will get you places.
A 2wd with a diff lock will get you most places.
A 2wd with a diff lock and high ground clearance will get you more places.
Driving on sand is more about tyres and pressures than 4wd versus 2wd.
Something like a 2wd ute with a diff lock is very capable.

I remember Top Deck and their double decker buses. I also remember seeing them broken down on many occasions and really smelly from so many people cooking and sleeping inside. No fun at all.

A four wheel drive with open diffs.... Well, assuming it is a part time 4x4 and you are in 4 wheel drive then it is a 4x4 - unless torque overcomes traction and a wheel stars spinning. It this happens on one axle then you are still going to move as the other axle is driving. Of course if a wheel on each axle has lost traction then you are going nowhere. In other words a 4x4 becomes a 4x2 when torque overcomes traction, so ironically a 4x4 becomes a 4x2 when you need it most i.e. stuck!

Same way a two wheel drive is not a 1 wheel drive unless a wheel is spinning out!

Gil

two wheel
 

Actually a 2 WD with one wheel spinning in a 0 WD :) unless you have a locker in the diff.
Equally a (part time) 4WD without lockers and one wheel spinning is a 2 WD, with lockers its a 3 WD

Cheers,
Noel

Price
 

If you can afford it go for 4wd, if you cant go with whatever you can afford. When I went to India in a 2wd I wished for a 4wd, not for road or track use but to get to out of the way places to camp. I got towed numerous times by other 4wd vehicles. I still got there and back though! as did many others including 2cv, Bedford CF, VW....

Graeme

for my tuppence,

no, you don't often need 4wd, crossing a Libyan sand sea recently I hardly ever used diff lock in my 90, but I am convinced I would have got stuck or bogged down much more often if I didn't have two driven axles to search for firm ground.

Although you don't perhaps need a lockable 4wd system in a car, 4wd vehicles perhaps tend to be built more appropriately for the terrain and for the demands you're going to ask of it.

Clearance, departure angles and sturdy suspension are important in some places, and most 2 wheel drives just arent built with that in mind. outright strength of the chassis is also important as well - Although old 405's can get a long way through a desert, the amount left broken in two littering the roadsides and pistes of Libya is amazing!