Nearly all the plans for workbenches that I've seen use a top which is laminated by gluing strips of timber together, even to the extent of ripping wider boards and then re-gluing them together. Why is that?
Carol Reed: Wood expands across its face. In workbenches, the face grain of the wood is glued. That limits the wood expansion to up and down rather than across. It provides a stable and flat work surface.
Mark Hensley: Stability. However, there are two trains of thought. Hardwood tops and softwood tops. Hardwood tops look pretty, but no one wants to screw or staple onto it. I would recommend laminating 2x4s together, then use a sacrificial piece of plywood on top. This method will allow you to screw into and staple onto your top and not worry about losing its "pretty" look. Then, on occasions when you have excess wood filler, you can fill in the holes on your top. One of my workbenches is built in this arrangement, and I have been using it successfully for over 23 years. Granted, the plywood top is now mostly Bondo.