If yes, how can you justify doing it when it most cases, it's theft?
Yes, but only in very rare circumstances with license confusion as I have multiple copies of the same add on licensed to different forums that get mixed up.
I have forums broken into their respective directories as a base to rebuild (to include backups) from the ground up if restoration fails with styles, templates, category configuration, etc. In this directory, I keep an /XF/ folder that contains that forum's XenForo licensed installation package as well as all add ons associated with it. I have to admit at times, during my organization, I have added add ons for other forums that weren't licensed to run it. I might've gone about a month or two without realizing the license issue, and when I do, I either disable/uninstall it, or purchase a new license to support the developers.
Some can't afford the price tag on the software.
And thereby have no right to use it. We can argue inequity all day and all night, but the simple fact is they do not have the right to use the forum software or add ons. This is taking money from developers, which legitimate users will be the first to bear the brunt of if they feel they need to increase prices to offset these lost sales, which is double-edged as it might force some to go the nulling route. If more had the mindset of copying add ons over to multiple forums, some developers that all of us rely on might call it quits... then what?
I suspect XF 2.4 has the structure (from
DragonByte's reply) that XF will either launch with add ons having the ability to do callbacks to ensure there's a legitimate license running, and if not, report back. Of course, this isn't solid, as I'm sure add ons will be nulled. But, you're also dealing with this at that point:
You don't know what they hid in there when they nulled the software.