I think a Plan B is important for an outdoor wedding (or any outdoor party). Even if the weather turns out fine, you will save yourself some sleepless nights in advance.
I wrote about this once before, I believe, on the old site. My cousin had a smallish (60 people or so?) wedding at a restaurant, with the ceremony in the courtyard and the reception indoors. During the ceremony, a few raindrops began to fall. Everyone pretended not to notice. But then it started to rain for real -- not a storm, but a definite drizzle. Eventually, the rabbi paused and asked if he should continue, wait, move inside, or what. The bride and groom fortunately have great senses of humor and just shrugged it off. So all the guests followed their lead and smiled and nodded that it was fine with us, too (I suspect that the cellist, though, was NOT thrilled!). I think my son, then a teen, called out, "Rock on!" A few of the guests grabbed some white tablecloths and stood underneath them for the last few minutes of the ceremony, which occasioned some joking about how we've all seen someone wear white to someone else's wedding, but we'd never seen anyone with the chutzpah to erect their own chuppa (wedding canopy).
The moral of that story is that everything turned on the bride and groom's relaxed reaction and who-cares attitude. Had they been upset about everything not being P-E-R-F-E-C-T, it would have spoiled the happy vibe. Instead, they made it even more fun and memorable. The ice was all broken, everyone was talking to everyone else and laughing on the way into the reception.
So a Plan B is important, I think, but even with a Plan B, some things just always don't go according to plan, and it's really important that the couple (and hosts, if different) keep a happy attitude no matter what, because the guests will take their cue from them.