Broctoon

I like exploring various options for bike cargo, but I don't think I'd use something like this Jack thing-a-ma-jig.

Here's why:

For less money and easier installation, you can get a bag that simply straps onto the handlebar. Boom. Done. And it doubles as a purse/fanny pack when you get to where you're going. Easily moves between bikes. Keeps your stuff protected from the weather. Keeps your stuff secure from falling off. (Not to say you can't secure things to a rack, but it's more complicated and time consuming than throwing them in a bag and closing the zipper.)

If you want to carry stuff that's bulkier or heavier than a handlebar bag can take, get a proper rack, with support struts that secure it to the fork down low. Ideally, these would attach to threaded bosses, but some bikes will require "P" clamps. This will handle much more weight with no concerns about strength/security.

When you consider the physics involved (really just geometry), there's a huge disadvantage to anything that attaches only at the top. Because leverage. I have a semi-rigid bag from Ortlieb with the same problem. Its pros include super easy and fast attachment/removal after the mounting bracket is initially installed; tough, waterproof, and very secure compartment; some nice features like magnetic top flap closure, zippered internal pouch, and key clip; and top quality materials and construction. One con is that it can't be shared between multiple bikes unless you spend additional money for more mounting brackets. Also, the bracket that remains installed on the bike is kind of chunky. The biggest con, in my opinion, is that it really doesn't carry much weight. This is entirely because of the leverage involved--a few vertical struts would make a world of difference.

I might sound like a hypocrite for saying I favor either a soft bag or a "proper" rack, and then saying I use an Ortlieb semi-rigid bag. But there are some applications where it's nice to have something in the niche between these extremes. The Ortlieb overlaps a bit into the realm of smaller and lighter bags and a bit into the realm of rigid racks. Although it requires some compromises, it offers the best of both worlds. A rack that attaches only to the bar and stem and is still just a rack (no enclosure built into it) involves too many compromises, for me. I see it as having the worst of both worlds.