There’s an enlightening article at The Register on Charles Davies’ (was Psion’s first employee) views on Symbian. He admits that the owner-licensees "chose not to have a common platform: it was common only from the waist down."
"Then we had Platform Security which was incredibly disruptive – more disruptive than people thought. And we did it in a way that broke compatibility, and said that would be the last time. It was a big kicker. We could have done that better. We could have done Platform Security in a less disruptive way. We underestimated the downstream disruption."
I have been a long-term critic of Symbian Platform security. As early as 2006, I was commenting on the problems. Even today, people have problems – I read on twitter that Jan Ole Suhr, the author of the well-known Gravity Twitter client was having problems because "The Browser Control doesn’t work properly without the Symbian Signed capabilites".
Platform security took difficult Symbian Programming and made it even more difficult.
UPDATE: There are even more problems now that Nokia allocate UIDs for use on Ovi.