Necessary? No. Worth the money? Yes, in my opinion. Loads of info here if you search, but my feeling is based on brakes. In 2wd, you have no front brakes. If you have a load in the bucket and anything happens to cause a forward shift of weight, you can effectively lose all brakes. There are ways around this, such as backing down hills and dumping the load, but 4wd effectively eliminates the need for such workarounds (while keeping in line with safe operating practices, etc.)
whodat
Edit: A note on Bird's reply, re: tlb's. When they excavated for my barn's footings, the contractor used a 2wd allmand tlb. It was totally unable to lift and move a load (in the loader) of dirt due to lack of traction; ended up with them digging with the hoe and me using my old, but 4wd, tractor to move all the spoils.