There's a guy that makes custom cargo cycles with a big box on the front. He makes those boxes where they do not lock. The reason is that if it's locked, people will break in and tear it up, even if it's empy. I think the lack of a lock on your bag is not an oversight, but just pretty much the accepted way to do things.

I can't offer an opinion on that specific bag, but I've looked at some others that would be comparable in cost (handlebar bags, specifcally). Ultimately, if it does what you want, it's worth it, and if it doesn't, it's not. Personally, I've not seen a "perfect" trunk bag. I'm using a little cheap fabric toolbox right now, and it works. The regular trunk bags are all padded, and the padding seems to take up half the volume. I'm not too concerned about waterproof, because anything in there that can be damaged by water will be in baggies anyway.