Do you judge albums based on how many tracks they have too? Art isn't necessarily about sheer quantity, is it?
It feels like this attitude devalues comics as a medium, meaning they're something that's only valuable in the present upon release as opposed to the work that will last for generations if it's quality is good enough. It's easy to get caught up in the week to week, but we have to remember that we're actually just a percentage of the total possible audience if the work is good enough to stand the test of time. Do you think it matters now that #12 of Watchmen was a couple of months late?
I can understand how frequent and extended delays can cause you to lose interest in a comic, especially when a new issue is so quickly read. I've experienced it with Berserk, waiting for months - almost a year or longer in some cases - for a new issue that takes fifteen minutes to read. It's just difficult for me to keep my interest in reading something in relatively unsatisfying fifteen minute chunks every few months.
I don't think there's anything wrong with getting bored with something that is taking too long between issue releases; just drop it off your radar until it either finishes or a new trade comes out or until there are enough new issues that you feel like you're reading something substantial.
I'm definitely not in the "per issue" camp, though. It's possible that an individual issue can be weaker taken in isolation, but still make the work as a whole stronger for its presence.