well some people, myself definitely included, can't really afford to buy things 'for the moment' and so they gotta somehow look at the future too to see what would suit their needs better. obviously if you can afford to buy lenses for the gear you have now, and repurchase them when your needs change, then good for you.
I think the point is that you actually don't lose much money (probably much less than you think, anyway) by catering to your current needs rather than hypothetical future, especially if you can find good used lenses that don't lose value at all. Some lenses work as well for both APS-C and FF, but if you plan to stay APS-C for a while, then you are probably much better served by, e.g., buying better suited EF-S zooms than more expensive wide/normal L-zooms that are heavier, somewhat wrong focal range, and don't produce much better quality on the APS-C than the APS-C-optimised EF-S lenses. It mostly depends on how much photography you plan to do before your FF upgrade.
Exactly - and for many, that FF upgrade remaing 'soon' for a LONG time. For example, the EF-S 17-55mm f/2.8 IS delivers better optical quality than either the 24-70mm f/2.8L or 24-105mm f/4L IS (when comparing all on a 1.6x body), and provides a true general purpose zoom (wide to short tele, whereas 24mm on APS-C is not wide angle).
As for losing money, I wanted an ultrawide lens before I had a FF body, got the EF-S 10-22mm. Sold it nearly a year later, after getting my 5DII, for a loss of $50 - and that was before the jump in lens prices earlier this year. If I sold it today, I'd have made a profit (after less than 18 months of ownership).
