Tinct (an alpha-blended sprite plotting module) was written because the Select alpha plotting wasn't made available to non-Select users. This meant that NetSurf couldn't render PNGs correctly and an alternative solution was required.
Flash forwards 5 years and various other applications also use Tinct due to the same lunacy that NetSurf faced. Except now, RISC OS Six supports a new 64k screen mode. A mode that Tinct, and thus any application that uses it, doesn't support.
Do you see the stupidity here? RISC OS Ltd's lack of foresight has created itself a dependency on a module that provides functionality that it already has. Given that the source code for Tinct isn't openly available, imagine if I'd turned around and had the same attitude as RISC OS Ltd., deemed the updates non-commercially viable and left the 64k screen modes somewhat of a dead duck for anyone wanting to use !NetSurf.
Also, such low-level stuff as this really can't be tested for and the used accordingly. For a start you'd still need to write the fall-back code anyway, so all you get is twice the code base to maintain. I'd also suggest that a simple look around will show that RISC OS Ltd. has no idea how to stop the OS stagnating and dying.
Regarding RISC OS 5's lack of such API support, it sits precisely alongside the support for 3.5 and later to me. Why couldn't RISC OS Ltd. have worked with whatever relevent parties and made it happen?