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One thing you can't get on subscription (or in some smaller stores) are back issues
You're going to want back issues if you start reading and collecting comics seriously. You don't want holes in your collection and, if you're a fan rather than just a hobbyist, you'll want to read the earlier stories first. Even with the fashion for retconning, there are themes and plots developed over a series; having the older books from a series fills in the gaps in more ways than one.
What constitutes a back issue though? Catwoman has been around since 1940 but, with her many revamps, those earliest stories have little or no bearing on the character as you see her today. Batman #1 is a collector's item more than a back issue. And a 'back issue' for a character can't include a first appearance in someone else's title. Can it?
The long running Cerebus the Aardvark can be said to have back issues as it is one diverting arc running over years. It is either the character or the series that can be said to have back issues, not the auteur nor the publisher. Which is not to say you won't hunt down back issues that feature a certain run on a book.
But it isn't just the vintage of a book that determines its back issue status. Archie Comics Digest is hardly a 'back issue' if its stories are stand-alone. It's not as if the characters age, so the emphasis is on gags, not development.
Generic titles and generic books confuse the issue as they have back-up or short stories that start and finish at different times. There are golden age cases of the feature character swapping with the fashion or the times, sometimes losing a place in the book altogether.
Generally however I'd class Valiant as having back issues.