Luckily enough I've visited theme parks with people I enjoy spending time with, so a bit of bad of weather has never "ruined" a park trip.
I can remember four occasions where I've been at a theme park and the weather has stopped me from playing.
Twinlakes
Was it this year or last? I can't remember. It was really cold, ice, snow, frostbite. Terrible. the coasters weren't working but thankfully there were enough indoor things to do.
Cedar Point 2007
It hammered down with rain and everything shut. We amused ourselves undercover for an hour until it cleared up. Yes, it was annoying that it chewed into ride time, but messing around with the like of Taylor, Snoo, Hyde, UC, Tom etc was more memorable than any ride would have been.
Goliath SFGAm media day, 2014
I managed two rides before the sky went black and Noah's Ark sailed by. Yes, I was annoyed that I never got a third or fourth ride, but taking shelter in the restaurant drinking free coffee and watching Darren wind up a gullible goon about Thorpe Park's MTDP was great entertainment.
Lakemount Park, 2014
When Darren, Jerry and I turned up, t was pissing down. We managed to get on Leap The Dips but the other stuff was closed. This I was most annoyed about because I wanted to go on my first Chance Toboggan but hey ho, it's an excuse to return.
So yeah, bad weather has never ruined a day at a park. It can be irritating but it depends who you're with.
There is a fundamental difference and mentality between European and American theme parks on the subject of weather. Some American parks won't open if there is a bit of drizzle mainly because American guests don't visit in bad weather. Europeans tend to go regardless and all you get from the park is a warning that the bad weather might make the ride experience slightly uncomfortable.
I guess my attitude might be different if I were visiting a park that I was unlikely to return to - say somewhere in Australia for example. But I realise I can't change the weather and plan around it accordingly.