Personally, with a game like this, I'd rather have a short campaign that gets you into the tools, gameplay, story, and so on without artificially drawing out the experience. Heroes is a campaign-driven game, because the AI is usually terrible unless it's scripted, and it's not really nearly as fun as a one-map sandbox as it is in a series of "levels." Even so, HOMM5 suffered from having maps that were clearly just filler, some of which were extremely long, arduous, and boring.
Consider, on the other hand, a game like Sword of the Stars, which is fantastic as a sand box, but which really could have used a clearly delineated introductory scenario. Much as I loved reading the manual, man did I want to learn that stuff in a campaign.
Consider, also, the horrid (no offense Brad) campaigns in GCII. I'd rather have a 5-8 hour, one map intro full of goodness than an arbitrary collection of maps just there to provide filler.