Yeah, it's a real revolving door during the season - the only constant is Krukow is always on TV as long as the game is broadcast by the Giants (so not Fox or ESPN). This is basically because of Dave Flemming.
Before Flemming came around Kruk and Kuip always did TV, except for the games broadcasted on local TV which were done by Jon Miller. On Sundays when Miller was doing ESPN Sunday Night Baseball, they often had a fill-in on radio. But because Flemming is the best broadcaster under 40 going, they needed to throw him a bone or two lest he move to another team, so he will do a couple innings on TV and radio in many games. He even called the last out of the 2012 World Series.
In the playoffs everybody's there, except when Flemming has other commitments. They usually all call a few innings, and Krukow does a strictly color role.
That takes a fair bit of magnanimity from Miller and Kuiper to cede some of the spotlight. The Mets basically lost Kevin Burkhardt because they didn't have a spot for him on either the radio or the TV teams, and I wonder if it ever crossed their minds to do some musical chairs so Burkhardt got to call games.