I, too, am having a little trouble with this concept. I had a list of the locomotives I wanted for my layout. Exactly 14 of them, no more, no less. I selectively acquired them at the best prices I could find, and I’m done now. If I had one that I wanted, I waited until I could find it on sale (although I admit that when IHC went under, I acquired two of their Command XXV line steam locos at fire-sale prices – as in $60 for a loco with an MSRP of $180 – but in the incorrect road name). Now, granted, you don’t have to have the same philosophy as I do, but I personally can’t see buying locos just to buy them.
And as far as restraint, I agree with Mr. Beasley (as usual). I have such a backlog of kits requiring building and painting, as well as miniature figures to paint, that I really shouldn’t purchase anything else until I get a little caught up.