It shouldn't really matter if you're flying in IMC or VMC with the hood up, you should be looking at the instruments
In the real world when you have an IFR departure out of a major airport it doesn't matter if it's VMC or IMC because you have to fly the instruments. It may be a bright sunny day but you still have to look down and follow those needles.
From a safety point of view I can understand why instructors want VMC days, often they are flying in an out of controlled airspace, in and out of control zones and climbing and descending into IMC, not always having a full Radar control service..
Will flying actual IMC for a IR student benefit them? Well personally I don't it will, flying with a hood up is just as good. Having said that, I've flown into IMC in the real world and hit turbulence so bad that we couldn't read the instruments from being bounced around, that will teach you something and no hood can give you that experience.