This has been a hard decision for me this year, because of the amount of rain we've had, the scarcity of trials in our area, and the distance we have to travel for trials.
Our local trial is outdoors, as is the next closest trial (4.5 hours away). After that, we're travelling between 6-8 hours to trial. I don't mind pulling my dog from the local trial if it rains, but it sucks to travel so far, and spend so much money, and then not trial. On the other hand, I don't like putting my dog at risk, so I try to balance the two out. It's further complicated by the fact that I don't usually travel alone, so deciding at the last minute not to go, or turning around isn't really an option. I did pull Starr from the Sunday of a rainy trial this year, and then had to sit around in the tent for 6 hours until my traveling companions finished trialing.
The biggest factor to me is that Starr doesn't like running in the rain. We do train in the rain, and I adjust my handling and expectations to reflect the weather, but the bottom line is my agility partner deserves a "vote".
So, from now on, except for our local trial, I won't be trialing outdoors. I feel bad, because I would like to support the trial closest to us, but it's not fair to my dog.