No overhead of creating addresses, you have an infinite address space for your account to play with freely.
More mail providers should offer such a workflow.
With aliases, I can drop the alias and you can not easily guess my real email address.
Just depends on what you want, or what you use it for.
With Fastmail sets this up automatically if you let them handle your DNS, though you can (obviously) disable it.
You're still correct though, your solution is more secure (and also what I use). It's always possible the form you submit automatically strips the +whatever from your email silently. I believe facebook does this (at least when checking to see if the email you're using has already been registered).
I moved to wildcard forwarder on my own domain so that *@mydomain.com get forwarded to my main email account. This gives me all the tracking I want and I can ban addresses if needed.
That still gets me the zero overhead and practically infinite addresses with none of the downsides of gmail--and I can easily change my main email account as an added bonus.