The latest portable version of Defraggler uses the Windows registry for storing it's settings.
This is not very portable. The other two Piriform products use .ini files.

...True, although the only information stored in the registry relates to window position/size etc.,
whether to check for updates and how often, and what language to use.

Very little information, and nothing that I would lose any sleep over: it's hardly
sensitive information !

I was more interested in the other advantage of portable applications: to keep the settings so I don't have to reconfigure when reinstalling or using it on another machine.

...Yes, I concur: this is one of the main reasons that I too favour "portable apps".
However, the window positions, sizes etc. may well need to be set per machine
basis anyway, and Defraggler defaults to English [which I presume is your first
language], so the only [slightly] significant things to consider are the update options.
I can easily live with that!

But [also] yes: it is nice when a "portable" app means *zero* use of the registry for
maintaining information. It just feels clean and fresh ! Long live the .ini !