Thank you for all the great info! In your estimation do you think resting the top on dowels in the legs would be sufficient? It's not nearly as cool as the approach in the video shown but much quicker/easier.
As for not necessarily needing the top stretchers, I definitely wouldn't see them as necessary if I were using solid lumber but I was a little more nervous about sagging since it is MDF.
Workbenches are a "lifetime" project, unless you decide that it wasn't what you wanted OR it didn't turn out as you expected. I have evolved through quite a few in my days, but here's what I have learned. The top must be attached to the legs in such a way that they con not move when a racking force is applied. The top is where the weight is so, given a sloppy or weak leg connection it will have "momentum" and be magnified.
There are two ways to prevent racking that I'm aware of:
One is to attach the legs directly into the top as shown in the video and in my model. There ain't no way you can rack them in a through mortise as shown.
A variation on that would be the use of stretches directly under the top securely fastened, but allowance made for the top to move in width. The legs are then attached in the corners with various means. My favorite method involves a corner brace first shown here by Lola Ranch. I liked it so well I started a competition to see who could come up with various ways to make it:
The second way is the use of a plywood backing panel attached to the legs and stretchers. Side panels will also help. Diagonal braces forming triangles, the strongest geometric shape, are far better than rectangles which are the weakest geometric shape and will collapse under racking loads. The rectangle is what the workbench is with the legs attached, so bracing is required. A plywood back panel is the equivalant of triangle bracing, it just doesn't appear that way.
Most if not all dressers and large cabinets have a plywood back panel for this reason.
Once you understand the physics of joinery.... forces that are in tension, compression, racking etc. you will be able to make projects that last a lifetime or more.
Some projects can be made very strong without the use of fasteners or glue, because the strength is in the joinery itself, mortise and tenon being the most popular and one of the strongest joints.