I think interleague play is great. The fact that each team only faces about 5 teams from the other league keeps it pretty exciting. It's like, "wow, the Cubbies are playing in Seattle tonight. gotta watch that, it'll be my only chance for a couple years." And it does seem to give a little bit of advantage to the pitchers. Since a lot of purists complain about expansion thinning out pitching and increasing HR's, this should decrease that concern a little bit.
As for previews of the World Series... it's not really all that likely that 2 teams that meet during the regular season will also meet in the Series, but even if they do, they've only played a few games against each other. More than anything, it just gives you some matchups to look forward to and kind of whets your appetite for the action to come.