While MV lamps can be used for growing, they should be avoided, for a few reasons that I know of.
1) They use a lot of electricity and produce a little light. They are not very eccicient. So if they use the same amount of electricity but put out less light, that means that they are generating much more heat. These lamps get very hot, which makes venting even more important then with HPS's or MH's
2) Even the improved spectrum one, put out a crappy light spectrum. While you can use them, your plant can not use the majority of the light spectrum.
So just with these two points, I would say if it can be avoided, then avoid it. With it putting out half of the light of an HPS and then half the output being unusable, it will not be much better then using flouros as for the light produced, and will get 10 times hotter then a flouro.
But these are just my opinion from the research that I have done. I have never used a MV before, but these are the reasons why. You could always try it, and please wait for a second opinion because maybe someone is or has used one, I am only talking from the research I have done on the topic.
I just found this article so I thought that I would post it.
A mercury vapor bulb is stronger than a fluorescent, as a 175-watt bulb will put out 8,000 lumens. However, it is still inappropriate for the later growth stages, as the light isn't strong enough for the large amount of heat the bulb generates. The topmost branches of the plants will get burned being in such close proximity to the bulb.
Unlike fluorescents, mercury vapor bulbs are not suitable for rooting as there is simply too much heat and the light spectrum is a very harsh shade of blue-white. This bulb is cheaper than the other lighting options, but its excessive heat production makes it very impractical. Thus, this lighting option is not recommended.