With the rash of injuries to important fantasy players so far this season, many fantasy teams have had to decide whether or not to replace the injured superstar. This isn't a concern in your standard Yahoo!-type leagues as these formats allow transactions up to game time and have a pool of players who are very likely to outperform the injured player over the span of the stay on the disabled list.
This is not the case in AL- and NL-only leagues and those leagues that have more restrictive replacement rules i.e once per week. In an AL-only, do you replace Joe Mauer if you have weekly transactions? Likely not as the dregs of the back-up catcher world make "barren" seem fecund.
But what if it was Ryan Howard? or Felix Hernandez? Can their short-term replacements accumulate as many stats as the hitter or pitcher would in the games his owner missed before re-activating them?
No doubt the fear of Howard returning and hitting three homeruns before you can get him active or Felix Hernadez throwing a complete game one-hitter is what makes the decision difficult, and what often motivates leagues to allow more liberal transaction rules.
However, I believe teams look at the potential loss and disregard what the replacement produces. If you pick-up a middle reliever and he happens to get a W in the 3.5 weeks that Hernandez was unavailable then you, at worst, breakeven. If you get a HR from Howard's replacement, you will also likely breakeven. (I'd prefer 2 HRs just to be safe, though.)