MH Lighting

I recently saw an ad for a company selling 175watt MH "retrofit" kits for
$75US.  Included is socket, mounting bracket, cord, plug, lamp and ballast,
and a one-year guarantee.  On a watt-per-dollar analysis, this would seem
much cheaper than a fluorescent setup with electronic ballasts, etc.  Two
of these would produce more than 4.5 watts per gallon for a 75gal tank, and
would make for a hood much easier to construct than one containing, say,
fl. tubes.  Where is my logic wrong here and what questions/issues should I
be considering?


P.S.  Just wondering. . . .  If PMDD ought, ideally, to be refrigerated and
kept in darkness, why don't the commercial products have to be . . . or
should they be and the companies just aren't telling us?