It's a contentious issue. In my experience it is definitely not necessary. I have found that what you may gain in higher intensity you may have to compensate for in lower duration, even if all other parameters are optimized.
Since planted tanks are essentially a spectator sport, lower duration is not necessarily that desirable; if for no other reason.
I use T8s (0.5w/l) combined with decent reflectors, and my plants pearl like mad and still grow at a phenomenal rate - even those that are supposed to require higher light intensity.
Also, lower light intensity gives you much more wriggle room if other parameters such as CO2, flow, and ferts dosing are not optimized; there is less likelihood of algal outbreaks, and nutrient deficiency etc.
I all but neglected my previous high-energy tank (with T8 lighting) for around 2 months and found that it thrived in much the same way a lower energy system would...
...and no algae.