Ideally, the question of "Timely vs. Quality" is one in which you have to select from two extreme choices when in reality you would expect to get a mix of both. This is much like when political figures, liberal or conservative, ask one to chose between "liberty or security". You should have both, and if times have come to the point where you have to choose one, then these are dark times indeed.
My magic answer on the topic is, "It depends". By that I mean, for some comic books, that SPECIFIC creative team is the main draw, and without it, it would seem almost pointless to publish the issue. One example is ASTONISHING X-MEN, where Cassaday's art is at least as big a draw as Whedon's scripts, or JUSTICE, where Ross' art is pretty much the main attraction. Fill-in stuff from another in these cases will not do. The audience will feel miffed and the book won't be the same.
However, for titles where the artist is NOT the main draw, but the character, title, or event is, then I feel the artist may not be as critically important. When a company has hyped an event for months, and almost literally every book they publish in continuity ties into said event, then you can say that the writer and the event itself are what sells; a nice artist is spiffy, but it won't make or brake the book unless you all but go out of your way to find an inappropriate fill-in artist.
Quite frankly, it boggles my mind that neither Marvel or DC seem ready to confront scheduling hassles serious, rather than with a collective shrug. If times and technology and technique have changed to the point that a monthly artist cannot be expected, on average, to turn in 12 issues on time, then plan the "fill in" art in advance. Attach 2 or even 3 artists to the book ahead of time, so that when one gets swamped, the other is waiting in the wings. RUNAWAYS seems to do this, as Alphona usually needs a 2-3 month rest after about 9 or so issues, before churning out the regular work again. Mark Bagley seems to have an uncanny ability to not only pencil 12 issues a year, but some 18, albeit some issues DO look "rushed". Is it because his style isn't as full of photoshop trickery and more akin to the 90's pinnacle, of which he is a part of?
Another part of it is that fans have got to give fill-in teams a fair brake. I remember when DiVito did two fill-in issues of YOUNG AVENGERS, some fans went hog wild. Yes, he's a shift from the main artist, but his art was still perfectly good and effective for the title, and both of his issues shipped on time. Going by the assumption that all fill-in's, by nature are terrible in every situation is inaccurate, extreme, and self-defeating.
Take ULTIMATE X-MEN. Obviously it is the title and the writer that sells that series at whatever time, as it has had an endless parade of artists. The artist usually changes about once every arc and during it's 30+ issue launch by Millar, we had fill-in art mid-storyline, and a lot of it wasn't good. However, some was; Finch's first art on that title was a fill-in issue towards the end of Millar's run, and then he'd return to do a year stint with Bendis. I could list a half dozen artists that have worked on that book, and that'd only be for about half of the series' run.
Something like ULTIMATES 2, where Hitch's photo-realistic art is about the same draw as Millar's "OOOoo, lookit me, I'm an Extremist Liberal Action Movie writer!" stylings, then a fill-in artist would almost be sales suicide. So I can understand just sitting on one's hands and waiting for Hitch's almighty 2 issues a year. Of course, Ultimates 3 is going to have Mr. Manga himself, Joe Mad, come along, which has to be all kinds of jarring if you think about it. Obviously there wasn't much effort to get a second artist whose style at least APED Hitch's (for my money, I think Duncan Fegredo could have done alright), which is actually rather telling because that means that sort of style wasn't as essential to the editorial board, hence why they got an artist whose style is all but a 180. It's like going from Maleev to Miyazawa.
