Many owners spend the fewest dollars possible on their bench hitters/fielders. The theory is that opens up more dollars to spend on the primary players, and bench players, usually having few AB's, are not as critical a factor.
There are several variables that affect this decision:
1) Number of high risk injury hitters
2) Number of needed defensive replacements
3) DH leagues vs Non-DH leagues
If you have high risk injury hitters, you will need workable bench players. preferably eligible at different positions. Not having effective bench players in this instance, can be devastating if the injury gods are unkind. With this equation, you will most likely need to spend more on your bench hitters to cover multiple injury possibilities.
The need for defensive replacements depends on your stadium and your team make up. If you are playing in Petco, are in alot of 1-2 run games in the later innings, having good defensive replacements for your big hitters--bad D type players, will certainly help. If you are playing in Texas, and your team is laced with power, probably defensive bench players are not as sorely needed.
In non-DH leagues, your bench hitters play a much bigger role, as pinch hitting is a factor, and can make a difference. It makes sense to spend some additional dollars to get effective RH and LH pinch hitters in Non-DH leagues.
So, given all that, how much a difference does it make?
I went back through my 2010 teams and tried to evaluate how much of a factor bench players can be.
A few things stood out.
In non-DH leagues, pinch hitting does make a difference. How much is specifically hard to determine, but I have several similar teams, that had different benches, and the teams with the better bench hitters did better. Looking at it, if I had to place a value, I would estimate that better bench hitters, in non-DH leagues, (vs the cheapest bench players available), can make a 4-7 game difference.
Defensive replacements, also hard to value, seem to have minimal impact, unless the players to be replaced are terrible fielders at the critical positions (CF, SS, 2B and C). Catcher's have several different factors (arm, errors, wild pitches, etc.) and a terrible catcher can permit the opposition to run rampant. As there is a limit to the strong defensive catchers in the 2010 game, getting a good defensive catcher is desirable.
The key to using the cheapest bench players in DH leagues, is to have hitters that qualify at multiple positions so that in the case of injuries, you can slide players around. Always good in DH leagues, to have at least one strong hitter on the bench, to have DH/multiple position flexibility.
The old addage applies though. You don't want to have salary tied up on your bench, unless it contributes to wins. There are some very cost effective bench players, but the owner knowledge of these players always increases over time, and they are harder and harder to get.