I think there's an important distinction to be drawn here between:
- freebies available on phpBB.com vs. freebies available externally
- freebies available on phpBB.com vs. premium themes / extensions available externally
The problem you'll find with freebies (which absolutely includes all submissions on phpBB.com) is longevity. This is evidenced by the number of contributions that start in the 'x in development' forums but don't make it to the CDB. Or, validated contributions where the developer doesn't provide support.
Developers who submit to the CDB on phpbb.com generally
do so for 2 reasons:
- As a philanthropic gesture to the community, as their contribution solves a problem or enhances user experience
- To promote an external offering via footer link (such as premium phpBB services or premium extensions / themes)
The promotion of external offerings is generally a good thing, as the developer now has a vested interest to keep their freebie alive and supported. I can attest to this through my own experiences, but also we can look back on Arty's contributions and his high-profile sponsorship with FreeForums.org (which later turned into self-promoting his own premium work).
On the other hand, philanthropic submissions (which is generally most of them) where the developer has invested time into creating their contribution, but has nothing to promote and expects nothing in return, are more of a significant problem. This is because the contribution's lifespan is directly relative to the developer's current interest / motivation level. Which could disappear overnight and never return. This is a risk that is equal in measure whether something was downloaded on phpBB.com or externally.
With regards to premium contributions that are only available externally: Generally speaking, premium phpBB contributions (themes / extensions) are superior in every way. Feature sets are superior (particularly in the case of themes / styles), code quality is normally better, updates are faster, support is better, and premium themes' aesthetics and UX are incomparable to any freebie you'll find on phpBB.com.
Particularly with regard to version updates or maintenance releases, phpBB.com submissions will be stuck in validation queues for many weeks, while external developers often publish updates on the same day of a phpBB release. On the phpBB 3.3 release, most of the best-selling phpBB themes on ThemeForest were updated to support it within a week.
Finally, it is important to consider and acknowledge that there are many exceptional phpBB developers (companies and individuals) who barely step foot in this community, notably Tapatalk, PixelGoose, RocketTheme (historically) to name a few.
All things considered it is absolutely not right to say that contributions should only
be downloaded from phpBB.com. If it's available externally and costs $$$ , you'll usually find it beats anything you'll find for free here