Multi-tabling online cash games is essential for someone looking to make a living from poker I'd say, as the additional tables mean slightly less variance. I.E. Crushing one table while simultaniously going broke on another may mean breaking even for the day (or better), where as taking a beat while single-tabling will be a real hit to your roll. Plus if your a solid by-the-numbers player, you'll see more premium and playable hands and (hopefully) win more cash.
For players looking for a bit of recreation its up to you, but if you are going to experiment with multi-tabling, drop down a level (even if it means playing micro's for a few days).
Multi-tabling cash games and MTT's can take the fun out of the game some days, and makes it feel more of a grind.
If you want to make real money
at the game then try it out. The matrix games at Full Tilt are a good jumping-off point. $1.25 for 4 identical SnG tables. See how you fare with 4 games with very little risk.