In conjunction with a specification (or documentation, if you prefer the more inclusive term), a reference implementation is a good thing. Without a specification, a reference implementation is an astonishingly efficient way of propagating bugs into third-party software.
The Second Life viewer is in the latter category, alas.
In a way, Mail Transfer Agents (MTAs – like Sendmail and Postfix) are somewhat like relics of our past.
They stand like great monuments to the baroque and arcane tasks that were once required to get an email from one person to another.
Like vast pyramids raised up just to inter some dead person, MTAs hint at the monumental complexity that used to be email.
Linden Lab maintains the source code for its viewer as an unencumbered asset (through the mechanism of contribution agreements). This allows it to maintain the value of the codebase in the event of a sale, but brings with it some rather obvious problems.
The latest build of the Imprudence viewer experimental branch is out now.