I've often thought about the same thing. I was carrying a 3 iron but I was not getting consistent results out of it. I switched to a hybrid (915 H) and it gave me much more confidence off the tee and off the deck.

Here are some of my observations:

First the hybrid is more forgiving and demands less from your swing. The hybrid has a nice sole that glides along the turf and wont dig the way an iron can/will if your ball striking isn't on point. Additionally, the fact that the hybrid has a face that has some curvature promotes a gear effect that can work to help keep shots on target the same as other metals in your bag. Obviously the 3 iron doesn't have this characteristic and wont provide any of that gear effect. In terms of the shot characeristics the hybrid is easy to launch high is very straight and can land softly to hold greens. This is the primary difference my experience with the 3 iron was that it launched lower with less spin and so I saw a substantial roll out. For this reason I primarily used it off of the tee on tight par 4s because I just didn't think it would hold a green. Ultimately my decision was to put the 21 degree hybrid in my bag but I agree with you that the new 716 T-MB is interesting. if the CG is low with the toungsten weight it will have that higher flight to hold greens with the look of a muscle back long iron that I like so much! 

I agree with you.  I play a 21 degree hybrid, an AP1 4 iron and AP2's after that. 

I'm going to replace my 21 degree with an 816 and I was debating on a hybrid to replace my 4 iron as well. Then I saw the 716 T-MB and thought that might be a better option. 

Decisions...

I'm also playing the 714 AP2s and 913 hybrid.  I just got a total fitting and hit all the new toys at TPI.  The 716-T-MB was amazing, but the 816-H1 is beyond amazing.  I couldn't stop hitting it at the fitting and my fitter, Robert, just started laughing saying, "you like that one don't you?"  Try both but the 816 h-1 is the one!

Leaning towards that.  I'm probably going 21 H1, 24 H1 and lose the 4 iron

Going to a fitter is the sure answer. But I believe if you're a really high swing speed (way up there) or if your launch and or spin is way too high then the T-MB might be your answer but 816 is most likely your sure bet.