Marvel has been shipping some of its books at a more than monthly rate for much of 2012. Some titles will have 15 or more issues a year. This means the books are coming out faster than the artists can draw them, necessitating either fill-in artists or teams of artists in a rotation. It also means that those titles lose out on having one consistent look (save Invincible Iron Man, which will finish a 50+-issue run with the same writer, artist, inker, and colorist later this year).
The best way to go about this is to have the artist changes occur at logical story breaks. One artist draws a complete arc of, say, six issues, then a fill-in artist does a shorter arc, then the first one comes back. It doesn’t often work out that way. Here [Chris Eckert shows how Jeff Parker’s Hulk series has done that dance well].
It’s worth noting that the whole thing takes a toll on writers, too. Ed Brubaker has been writing Captain America for several years now but couldn’t keep up with Marvel’s demand for a faster schedule. Cullen Bunn had to be bought in to co-write the penultimate storyline before Brubaker’s final issue. Maybe it’ll be good, but it’s a shame his long, highly-regarded run has to end that way.