I would say it is not neccessary. The amount of L-arginine that you take with your meals is a enough for a "normal" man. That means, if you're not a profi bodybuilder.
Supplements are never necessary, they are simply a supplement to your training. Anything that will help is a benefit being pills (VIAGRA, CIALIS, some [words=http://www.mattersofsize.com/join-now.html]Penis Enlargement pills[/words], arg., etc.) but they are in no way a substitute for hard work. My logic is if you can afford it, add them in, if not, train harder