While I can see the reasons of both ends for cover artists (interior artist slow or not "flashy", fancy cover artist to draw in the reader to buy)  from the companies, I really enjoy the diversity found with other artists doing covers. Marvel having Kane doing most of their covers provided a solidarity and stylistic uniformity and you also got to see Kane's version of heroes and villains you might never see him doing on interiors. It was great to see Kirby on Avengers and FF covers knowing he helped originate those teams and likely we would never see him on the insides ever again. It was a blast for me seeing other artists renditions on covers, especially the likes of Steranko, Gulacy and Adams in the days of the 70's and later on seeing Byrne and Perez on covers for other series. DC on the other hand allowed you seeing the classic old timers like Cardy, Infantino, Andru alongside Buckler, Garcia-Lopez and others on covers as well. Neal Adams during his run as cover artist for DC in the late 60's/early 70's provided a uniform look and style which captured the younger generation's curiosity with the more plain (or mundane to some) interior artistry. 

Either way, cover versus interior you had a choice to make. Seldom did I ever purchase a comic book "just because of the cover" but there were some times it occurred (how I got hooked on Killraven, Adam Strange and some others) and there were plenty of times I  would buy a comic because it was part of a series I followed and liked even though the cover might have only been so-so. In the end all that matters is garnering sales, which is a moot point these days. If having Mignola or Art Adams or Brian Bolland doing a cover will guarantee a few extra copies being sold, then go for it. Just quit the dagnabbit multiple covers for selling comics, it isn't about the comic anymore, it is about the cover alone generating the sales minus any caring about what is in-between the covers...