It's really only necessary if you require strength in a specific range of movement, so for armwrestling you want to be strong in the range you use on the table, however if you are looking for complete development of the muscle it's best to use full rage of motion. I try to do a bit of both, when i get down to 85 i'll train to gain muscle with a full range of motion but adding in short range heavy work to add strength to the new muscle fibres, giving me bigger, stronger muscles.
The best advice I can give is find what works for you and stay with it, just change things up every now and then to keep the body guessing.