I have a spare on all my trailers, but I know there are SO MANY people who don't.
In my younger (dumber) days... Over the years of dragging jet ski's, and dirt bikes... I lost 2 tires. Needless to say... I didn't have a spare. In turn... it basically ruined the day, since I would then have to track one down. Now that I have them on my trailers... I haven't lost a trailer tire in 20 years.
Like you, I have a spare for every trailer I own. This will ensure that I will never need it. But if I didn't have it, I'm sure if have a flat every trip.