David Brothers has a very excellent post about how the notion of the stable creative team has gone out the window at Marvel with their double shipping madness. I think there's some more pernicious impacts as well that David didn't address.
One primary consideration is that every book, every where, (nearly) every time, suffers from what we generally refer to as "standard attrition" -- that is to say that virtually every comic shows a slightly decreasing audience each and every month as readership walks away, or gets sidetracked. There are certainly ways to get bunches of new eyes looking at a book (crossovers used to do it... not so much any longer), and spike those numbers up again... but the general rule is a slow steady leak in all serialized entertainment.
What happens when you produce your comics twice a month is that the decreasing demand curve does nothing but accelerate because you're giving readers more opportunities each month to "jump off"; and, in fact, you're making it considerably easier TO jump off, because it is that much easier to get behind.
I can't say for certain about any other store, but at mine, while I sell the most DOLLARS to every-Wednesday readers (thems tending to be the most voracious of consumers to begin with), there are way way way more readers (at least 3:1, maybe as much as 5:1) who get to come in once-a-month maybe?If you're lucky 15-16 times a year, but many more can only can get in 4-5 times a year.
A corollary to this is that I rack the last 3 issues of any given series, and this is how I have operated for 23 years now. I'll break this pattern for things that are hot and are continuing to sell well (I'm still practically selling sets of BATMAN #1-6 almost every week, for example), but that does NOT describe most Marvel comics today. What THIS means is that if issues are put out every 2 weeks, instead of every month, then any given issue is only going to be able to be displayed for 6 weeks, rather than 12. This, too, lowers sales.
Finally, people are talking about not being happy with what they're being offered. Marvel hears this as "only wants to buy titles that 'count'", and, certainly, that's PART of it... but the reason people don't believe that books "count" is because there are so many of them -- what's going to happen when they double ship everything is that the audience will then decide that "X-MEN" "doesn't count", because they want to buy just UNCANNY and WOLVERINE &... People (most people that is... there are always exceptions!) simply aren't buying a franchise, at the absolute outside, at two books a week. Most franchises can only max at a single book a week, and several can't muster better than fortnightly.
If you overship of what people WANT to buy, then customers are going to pick and choose, and THEY are going to decide what "continuity" is right for THEM. You, the company, don't get to decide that (though I get that you think that you do). What's especially dangerous about this is that much of the mechanism that keeps Marvel going is the "Marvel Zombie" effect -- sort of a specialized "gotta have them all" syndrome. But Marvel has, over the years, in a completely self-inflicted fashion, driven their most hardcore customers to have to focus on franchises, rather than the Marvel Universe as a whole, and then down to characters -- we've gone from "Marvel Zombie" to "Avengers Zombie" to "Iron Man Zombie" (or whatever) almost out of necessity. To break these habits, to bring this buying inertia, is to harm the very loyalty to the brand that made it great.
Especially when you're now asking $4 for (in some cases) 19 pages of content -- well doesn't anyone up there see that's exactly the wrong time to be fragmenting brand loyalty?
Marvel's Market Trajectory is dire now, and here's my anecdote of how bad I think it is: umpty-years ago, when Marvel went exclusive with Heroes World Distribution, therefore setting off a set of dominoes which have almost everything to do with the market's current state, I decided the only real protest I could make was to not carry Marvel comics on my racks. I would carry every preorder that people wanted to place, but I was unwilling to spend my own money on something that was clear was absolutely against my best interests. There was a ...15, 16 month period, I guess?... where Marvel comics were officially "subs only" at Comix Experience (our sales actually increased during those quarters as well, which I found interesting -- I attributed it to more rack space being given over to other books; expose = sales.)
Anyways, today in February of 2012, I'm selling fewer copies of AVENGERS and UNCANNY X-MEN, including both subs and rack sales than I was selling of AVENGERS and UNCANNY during the Year-Without-Racked-Marvels. That's CRAZY. I had like 32 subs alone for UNCANNY back then -- now I can barely sell 20 copies TOTAL of UNCANNY. Because UNCANNY is $4, and because it ships more-than-monthly, I'm grossing more dollars now than I was then... but it's too a tiny audience, with little prospect for growth... because it is $4 and more than monthly.
The best-selling Marvel book was under 65k -- unless something drastic changes in management of the line, the regular ongoing monthlies are just going to keep dropping from there simply because Marvel asks far too much from readers.