Steve - I wondered about that, too. I heard that you were supposed to do low reps of the highest weight you can handle to failure, so that's what I've been doing.
Well not to go off course here, but I think the OP will get something out of this so.....
There really isn't a right and wrong and what is considered optimal will be different for each individual circumstance.
For instance, more than likely high volume programs like those usually associated with light weights and high reps tend to expend more energy. That said, these can be optimal for people who have a lot of fat to lose since caloric expenditure is the primary concern in order to get the excess weight off.
But as you move along the spectrum towards leanness, volume of training and caloric expenditure becomes less of a concern and muscle and strength maintenance becomes more of a concern.
Follow me?
Here's two examples to solidify what I mean by this:
I'm currently working with two women who are extremely obese. They've got the issues most would associate with anyone carrying around hundreds of excess lbs..... decreased mobility, limited strength, poor cardiovascular health, etc, etc.
I'm not going to program any sort of complex exercises with heavy training for these women as it would be pointless. a) losing muscle isn't a concern at this stage in the game for them. Heavy training best stimulates muscle maintenance so it's out. b) complex exercises like squats for example are pretty much impossible as their center of gravities are so off beat that they'd fall over. c) again, i'm in this for caloric expenditure for these women.
Then for the opposite end of the spectrum, take someone like myself for example. Losing fat for someone like me is very different than losing fat when you're someone like the women above. We are not biologically hard-wired via evolution to be lean and musclular.... quite the contrary actually. So when someone who is already relatively lean is looking to get leaner, muscle loss is a likely probability if you don't take the necessary actions. One of these actions is more of a strength-oriented focus with regards to your resistance training program.
In these latter cases, I don't use weight training as a primary means of "metabolic disturbance" from the get-go..... it's not about the caloric expenditure the bout of training provides. Rather, it's about the signaling the bout sends to the body. The caloric deficit can come from other mechanisms (i.e. cardio and diet)
Basically the leaner you get, the more sensible and economical you must be in your approaches.
I'll add that it's usually not an all-or-nothing proposition though, keep in mind. Most of my routines have a combination of strength and volume work.
You also made a comment about going to failure.
Going to failure too often is a no-no.
This is even more true when dieting.
Lifting to failure causes a lot of disruption in the body and while dieting, recoverability declines. This is tied into that economy of choices from above.
I could get pretty technical with the whys and I'm not too sure how to explain it without doing so. Suffice it to say though, that one should avoid maximal efforts, including reps to failure, except on an infrequent basis.