The purpose is to guide and keep the blade straight. If you look at a commercial quality saw you will not see rollers, you will see large square steel, ceramic or even wooden blocks. If you compare the blocks to a bearing your going to see blocks have many many times the contact area guiding the blade. A roller bearing has probably .001" of contact with the blade times the width of the bearings. My 30" American has a 3/8" or 1/2" wide hardened steel blocks. There is just no comparison in the contact area. While the bearing reduce the friction, so what? It's not a problem.

I have heard but have not read it, that one of the Magazines did a review of some of the commercial guides for sale and concluded that the Cool Blocks were the best choice and better than bearings. Many saw used to come with and still are being used with oil soaked wooden blocks. Again, larger contact area and they work fine. My saw had wood blocks on the bottom and steel on the top. Once I get it up and running I am going to put those back on there. Easy to make, to replace and you can't beat the price.

If you ask on the woodworking forums you will get a lot of opinions and most will say they are best thing since sliced bread. Not to say that they don't work, they obviously do guide the blades, but I don't think they are an improvement and don't improve anything. Many people 'upgrade' to them but they didn't have a problem to start with. They just think they need them and it makes them feel better. If you press them on how it improved the saw you will never get a good answer. Now if you have a problem and they solve it great! I have seen one or two people that did have a problem and these solved it, but for the most part it was just an "I WANT IT" purchase.

All that to say, if what you have works, keep the money in your pocket! if you do need guides I would look at blocks, not bears. It's your money obviously and you can do what you want. ;)