I agree with Cleveland wrt pitchers over hitters (assuming 4-5 bench spots). Here's why:
In a normal week, your players will play on Wed and Fri/Sat/Sun. Most players will play on two days of Monday, Tuesday or Thursday, and sometimes all of them. Occasionally you will have 2 days off in a week, but this is very rare.
If you have a starting batter, he will play the majority of those days. If he's given a day off, you usually won't know well-enough in advance to adjust your lineup (they are usually game-time decisions unless you're following the local media). The result of using 3 of those 4 or 5 bench slots for batters means you'll have 3-4 people that you MIGHT be able to cycle in for one day a week, plus maybe a "day-off" fill-in or a great match-up (like batters playing in Colorado), giving you at most 5 games more a week. On average, you'll see about 9 X 6 = 54 games... so those 5 extra games might give you about a 10% boost to your Runs, RBIs and SBs, being VERY generous (since (1) it is possible those bench players will also have a day off the same as the player that they might replace (2) they may not have an advantageous pitching matchup (3) they will be of lesser quality than those your starter and (4) you often won't know when to put them in since you won't know about individual day's off in advance).
Compare that to adding 3-4 extra starters (and using 1-2 extra batters that fill in for DTD injuries). If you have a typical team with (for example) 5 starters, an extra 3 starters will give you (on average) about 4 extra starts per week. The 5 starters you have will put up about 6 starts. That means you'll have 10 starts with 3 extra starters compared to 6 without, or a 67% increase in the number of starts. That will improve your chances at Wins and Ks.
The only exception to this is if you go with a strategy of 4 solid closers and 2-3 good starters. In that case, you will be playing for WHIP, ERA and Saves each week, and Ks not so much. However, getting 4 solid closers is very difficult in a 12 team league. If you are going for WHIP & ERA, quality is more important than quantity, and you could use your extra slots on fielders or long shots (that ride the pine until they pan out). Even then, I would rather have 3 prosepcts / flyers than 3 extra batters who will play once every week to two weeks...