Yes, the trellis is for ease of eventual harvest, pruning, and confinement, not to strengthen the plant itself. Several plants will eventually be sprawled all over, unless you prune their branchings very short, then a stake per plant would do.

Even the erect (not trailing) blackberry should be supported for a more healthy plant and for better picking. For easy and cheap support, just anchor down some metal posts and run two of three wires around the posts. You can get all supplies from home depot or so. They do not have to be the wood T posts. But they do look nicer that way.

I would make it attractive to look at. My grandmother used dead limbs to create her blackberry support trellis. She sank an eight-foot 4X4 treated post into the ground two feet(and used cement into the hole since she used it many years) at each end. Every six feet she placed an up-right limb hammered into the soil about 6 inches. Then she tied limbs from the end poles to the up-rights with wire every 2-feet of height. It had a homey, yet organized look in her garden.

Thanks everyone.. what about growing near tomatos, strawberries, etc...?

The plants like each other, not a problem.

Well, not exactly. There are risks with planting blackberries near tomatoes, strawberries, peppers, potatoes, and several other vegetables. You can read about that here: Verticillium Wilt. This disease is prevalent across the US, not just in California.

Yes, that is true, but it is more a problem replanting. You don't want to plant tomatoes where you grew tomatoes the year before either. So don't grow tomatoes and put raspberries there the next year. It's good to know these things, thanks for mentioning it. As long as you plant them next to each other and not together, I don't see it as an issue. And next year you will need to grow something else in the spots. Well you can leave the strawberries. My raspberries keep invading the strawberry bed which is right next to it. I would not grow caneberries there, but next to it? Sure.

One does need to rotate crops. I do every year. With caneberries you can't, but I grow all mentioned right next to my caneberries. I just rotate them in 3 different beds. All next to my brambles.