In Maryland, it is NOT illegal to operate during daytime with high beams. I suspect it is not illegal many other places (if any) either. It is illegal to dazzle oncoming traffic with lights.
That said, I always now operate during daylight with my high beams on. I found that for some reason oncoming, turning, and cross traffic acted like I was invisible when I operated with my low beams on the GL1800. I didn't have the same problem on my previous ride, a Shadow, which had a single headlight. I think, maybe, the two headlights confuse oncoming traffic into thinking you are further away than you are. Since changing to always driving with my high beams on during daylight, I don't feel as invisilble. I think it is much safer. And, come on...in the daylight, the high beams don't blind or dazzle anyone. If you think they are too bright, set them in a lower position.