The thing you have to consider is that if you don't offer a TAPI interface
what alternatives do you have?

CSTA is one option.

Another option is to provide a proprietary interface.  for example, Mitel
provide an SDK which developers can purchase and develop CTI programs - but
obviously they will only work with Mitel PBX's.

It depends where you anticipate your market.  If your PBX is destined to be
used in call centres then CSTA could be a good option.  If your PBX is
general purpose then I would advise TAPI is the way to go.

Or if you want to make the interface to your PBX closed then a proprietary
interface.

The proprietary interfaces tend to be C API's.

Note that none of these options tend to be .NET friendly.  .NET developers
would need a wrapper around all these interfaces.  As would Java developers.

There is often a disparity between what developers want (flexibility, easy,
low cost) and what PBX vendors want (making money out of developers, tie
developers in, etc).

Angus