Yes, you're right, it's really a "pay me now or pay me later" type of situation. You're not actually saving any time, you're just moving around WHEN you spend the time.
In my case, however, the reason I sanitize before storing is because I give the carboy a good shake with the Star-San to make sure the entire inside is coated in bubbles. Then I pour out the excess. But there are still quite a few bubbles in the carboy. When I seal it up and leave it for several days, eventually the bubbles all go away and I'm left with a small puddle at the bottom of the carboy that I can pour out right before filling it with wort.
If I instead sanitized on brew day, those bubbles would still be in the carboy, meaning an amount of StarSan equal to what I would normally be able to pour out will instead remain in the carboy and mix with my beer. I know they say "don't fear the foam," and I don't - it's not like I rinse it out or anything. But if I can keep an extra 2-3 ounces of StarSan from polluting my beer, without sacrificing any degree of sanitization, why not do it?