Healthy raspberries can grow to 8 feet so mine are in a row with three parallel wires strung between posts at each end. I could actually do with four wires. As the canes grow they're woven in and out of the wires and form a narrow hedge. Fully laden raspberries will bend over, brush the ground and blow about. It's also much easier to harvest from trained raspberries.

Yes, raspberry canes that become tall are more likely to kink and bend in strong breezes, especially when laden with berries or when wet. Even a landed bird could be the last straw. Most blackberries are far sturdier. When caneberry tips touch the ground they can take root under certain conditions and that is another reason to tie them upright.

This is my setup in May -- harvest would be a messy affair without the trellises that keep the canes upright. They grow to almost 6' tall.