Here's a good preview of the Giants-Reds series from Chris Quick and Chad Dotson of Bay City Ball and Redleg Nation.
The only surprise here is the Giants went with 12 pitchers rather than 11. Considering Cain and Bumgarner should be expected to pitch deep into their starts, carrying eight relief pitchers seems a little excessive. The Giants used an 11-man staff in winning the World Series in 2011.
Having three lefties in the bullpen -- plus Zito, if he doesn't start a game (the Giants haven't announced their Game 3 starter) -- will allow Bruce Bochy to play matchup games with Joey Votto and Jay Bruce, the only two lefties in the Reds' lineup. With so many relievers, I guess Bochy could go to a left-right-left-right strategy late in a close game if desired.
The Giants managed to rank second in the NL in runs per game in the second half despite hitting the fewest home runs in the league. They did this without much production from left field post-Melky Cabrera. Blanco and Nady platooned, but Blanco should get the starts against the Reds' all-righty rotation. That also pencils in Belt and his .380 OBP against right-handers at first base. Posey did play quite often there (Sanchez started 49 games at catcher), but the matchups seem to indicate Posey will remain behind the plate the whole series.
Huff is the only lefty off the bench, but the Giants won't do much pinch-hitting except for their pitchers, the Blanco/Nady platoon and possibly shortstop Crawford late in a critical situation.
The Reds have gone with 11 pitchers and 14 position players, but the problem is two of those position players are redundant, not to mention without value. Cairo and Valdez are two infielders who can't hit (Cairo hit .187 and Valdez hit .206, both without power or on-base skills). OK, you need a backup infielder, and at least Valdez ostensibly carries a good glove. Backup catcher Navarro (selected over rookie Devin Mesoraco) hit well in 69 at-bats, but that's a fluke; he can't really hit either.
It's not that Reds' bench is a complete waste -- they'll have Rolen or Frazier available, depending on who doesn't start at third base; and Heisey at least isn't a complete waste. But the only lefty bat off the bench is Paul, a career .259 hitter and also the best wheels (although the Reds don't have any obvious candidates to pinch-run for except maybe Hanigan). It's unfathomable that the front office didn't acquire another left-handed bat to bolster the bench. There's absolutely no reason a playoff roster should have Cairo and Valdez on it. Heck, you could argue pitcher Mike Leake -- who hit .295 with two home runs -- is a better hitting option than those two.
The rotation and bullpen speak for itself. Dusty Baker won't really have to worry about too many matchups. Posey crushed left-handers, so you won't see Marshall facing him in a crucial situation, but that certainly won't preclude Baker from using Chapman, even if Posey's spot is due up. Switch-hitter Sandoval has more power from the left side, so you may see Marshall used to turn him around. Belt is another lefty the Reds can gain a significant platoon advantage against.