It's pretty simple from my perspective: buy what you love, or at the least what you like a whole lot. If you happen to have really deep pockets, then go for it and get a whole mess of great decks. If you're like the other 99%
then get a few here and a few there. You don't even have to get the super-rare or the ultra-rare; there's so many different (and excellent) garden variety basic brand decks out there that cost less than $5 a pack that you could spend a lifetime getting them all.
I enjoy my cards. I open at least one pack of everything I get - and whenever possible, I get at least two packs. It's less for the resale thing, though I have done a bit of trading in the past, but more for the "I've had a fascination with playing cards since I was a kid and now I'm learning to be a magician" thing. Have you ever seen a more versatile yet inexpensive form or entertainment in your life? Countless games, and a cheap but decent deck is only a couple of dollars! My extra decks? I consider them backups for when the open pack wears out or gets a torn card or something like that. I'll open anything - I have an open pack of Gold Arcane, Red Artifice, Black Ghost 1st, even Broken Spell (THAT'S a hard deck to find, I tell you). I'm working on getting a White New Fan Back - which I'll open the moment it arrives. Buying something simply to watch it attract dust seems empty and hollow to me.
My humble opinion: don't by a deck simply because it's rare. Playing cards are all "rare" - as a collectible, they're categorized as ephemera; something that was never meant to last, "ephemeral" in nature. Show me a garden-variety Bicycle Rider Back pack from a hundred years ago and I'll show you something that's extremely rare, even though they were made in the oodles of thousands. Get a deck because it's really cool-looking or reminds you of something you like or shuffles like silk covered in butter, not simply because there's only X number of them in the entire world. If you collect simply for the sake of "gotta get 'em all," you'll end up broke and miserable sooner or later, or you'll get frustrated and quit the hobby.