Good point, using an MH (or T5 fixture) pays for itself that way.
Perhaps that is merely something from the past when HPS (streetlight) bulbs spectrum was of less quality than nowadays. It differs per HPS bulb too, I use the 400w philips son t 'agro' for vegging for example:
"Agro lamps are suitable for flowering as a standard HPS lamp, but are better for growth. These lamps have 30% more blue light in the spectrum and are developed by Philips especially for horticulture."