Also Angstrom has been around for years, and was stable well before the Pandora was conceived, whereas the Debian based distros are quite recent. We couldn't use Debian because it didn't exist at the time, our only choice was Angstrom or a similar tiny linux. There has been a debian release and you are perfectly welcome to use that if you so desire, but I believe for the time being they are correct in continuing to use Angstrom: it is stable with known compatibility, if they switched now it would require a lot of testing and who knows how much porting to ensure that everything continues to work exactly as expected.
Also, your understanding of the PND system is incorrect. The way they are setup is in fact more secure since they're sandboxed from the rest of the system: "installing" a PND doesn't touch the root filesystem at all unless you allow it to (and it asks for root password when doing so so you can tell and make an informed decision). Helps control your installed applications and prevents malicious PNDs from doing damage to the firmware.