I rationalize it by not spending more than a certain amount regularly on SW figures. For great figures of characters I'm wanting, I pay extra, for mediocre figures I only buy them cheap, and I save money for other things at the same time.
The best Han Solo figure I've ever owned, at least articulation-wise- Carbonite Han really beats him for good sculpting- is Cantina Han. A super-articulated ROTJ Han Solo with removeable accessories including a good-looking trenchcoat is just too good to pass up, even at $13. My best X-Wing gear Luke is a paint-worn POTF2 Luke, who always looked not only buff but suspiciously like he was wearing snowspeeder gear rather than his X-wing gear and had white gloves and boots. So an ultimate renedition of X-wing Luke is also very compelling.
At the same time, I skip a lot of figures I don't care for until they're cheaper. I kind of wanted a Bib Fortuna and a Major Bren Derlin, but y'know, I just haven't cared enough to buy them even at Wal Mart prices yet. If they stick around long enough that I can spare $6, maybe.
And then I buy old things and often do a little customizing and wind up with a figure I like. The best Chewbacca I own ATM, though I plan to shell out a gazillion $$$ (well, $$ maybe) for a VOTC Chewie eventually, is my repainted Falcon Mechanic Chewie, and I got him for $4 and the paint was probably worth about $.30 total considering how much I actually used. Toss in the crossbow from my POTF2 Chewie and the removeable silver chain I'm planning to make for him, and who needs a TSC Chewie?
It's all a matter of perspective, I guess. I do put a limit on figures I really want; after all, I haven't bought Mara Jade at her current $20 price tag yet.
Personally, I understand Army Building as a general concept, but I'm not really itching to do it myself- I just want enough passable troopers to fill in around my main character figures, and one really nice representation of each kind of trooper. Eventually I hope to have an army kind of like this to fill my display:
10 Various Clone Troopers
1 VOTC Stormtrooper
1 modified Evolutions Stormtrooper
3 Commtech Stormtroopers
2 POTF2 Stormtroopers
1 TSC Tie Fighter Pilot
2 POTF2 Tie Fighter Pilots
1 TSC AT-AT driver
1 POTF2 AT-AT driver (probably repaint the boots)
1 VTSC Scout Trooper
2 POTJ Scout Troopers
1 TSC Sandtrooper
1 Evolutions Sandtrooper
1 POTJ Sandtrooper
1 Real Snowtrooper That Hasbro Hasn't Made Yet
3 Saga/TSC/POTF2 Snowtroopers (whatever I can get)
Out of those 31, I have 10, and none of them the very best versions; I probably won't collect all that for a couple years. But when I get my hands on somethig really awesome, I really enjoy it and play with it a lot, so it's worthwhile to me to try to get it, and since Scout Troopers are something I have none of, something I've wanted for years, and there's an ultimate version in stores/going to be in stores soon at the lowest price I'll probably ever get one for, it's my best chance to grab one I think.
It's all a matter of your perspective and wants, though.