I have my TT camper tires balanced, but I don't bother with my landscaping equipment trailers.
I tow my TT at 65mph routinely. My equipment trailers are usually local, and usually 40mph or slower.
Years ago I asked my local shop to balance my TT tires... he said they don't bother balancing trailer tires. I asked again, saying that I tow it at 65mph, so I don't want vibration to be an issue. He said "hmm... I never thought of it that way".
I don't think it is a critical issue at all, but for the minimal cost (usually included with new tire or maybe $10 each) it makes me happy.
Why balance motor vehicle tires? Mostly because of the vibration at higher speeds. You can travel all day long on mildly unbalanced tires at 40mph. Go 65+, it gets annoying real fast.