Incumbents in office have experience that voters like. That politician likely already has connections that will make his or her job easier in the long run. Incumbents do last longer in a two-party system because that person has fewer opponents to defeat in an election, but that isn't necessarily a bad thing. The only downside is that an incumbent gets used to their job and may become complacent.
No, it is not good that incumbents last longer in a two-party system because it encourages politicians to game the system to their advantage, not to serve their constituency. In a different sort of system, preferably multi-party, such gaming could not occur, or at least not quite as easily as it does now.