Ave. Funny guy. 
I started using generics quite some time ago. Eric Johnson (guitarist) started this thing back in the day, because he claimed he could hear the difference between Alkalines and Carbon Zinc batteries in his distortion pedals. Well, when you're a tone junkie, you can't let this slide and you have to try it for yourself. Well, I'm one of the people that felt that there was a difference. There has been endless discussion about it in guitar circles, but the primary idea is that the carbon zinc batteries have a voltage sag which affects the circuit. It's subtle, but there is a difference. It's supposed to be much more pronounced in germanium based pedals than it is later transistor distortions and compressors. It's even given enough merit that boutique pedal makers actually recommend carbon zinc batteries over alkaline.
Short story long, after dicking around with this battery debate, I found that pretty much any battery was lasting as long as any other, so I stopped buying Duracells and Energizers and just went with the cheaper stuff. And in my regular alkaline use, I've found that Ray-O-Vac alkalines last just as long as Duracells and Energizers and are pretty much half the price.
There's definitely something to it. A lot of pedals won't care until things start clipping that shouldn't, but the old fuzz pedals can really be quite sensitive to operating voltage. I should say that part of my business is actually building pedals.
Anyhow, I include Duracell ProCell 9Vs in my product for a number of reasons:
1) they're much smaller than cheap, crappy batteries. They fit better in smaller enclosures.
2) The intercell connections are spot welded AAAA batteries. Crappy batteries either have pile construction, which is really piss poor all around, or connections that are mechanically held together. I think Energizers are just held together by the outer shell...take it apart, and the whole things falls apart if I remember correctly. It's been a while. Take a crappy, pile style battery and squeeze it. The voltage will change. I think Dave may have even shown that in a video. I've taken apart a lot of batteries when I was selecting one, and there are huge differences in manufacturing quality and design.
3) Can't remember ever having one leak. This one's a biggy. All batteries are definitely NOT made alike in this regard
4) the ProCell version is the same as their regular batteries, but they're available in bulk with individual terminal caps. Makes shipping them in the unit very easy, and if someone doesn't want to use a battery they can take the cap off the cell and put it on the snap terminal to keep that from shorting out. They don't cost much in bulk. I pay just over $1 each in cases of 216.
Lots of advantages to the Duracells that don't really have anything to do with voltage or capacity.