Why do people tend to mix deprecated fixed-function pipeline features like the matrix stack, gluPerspective(), glMatrixMode() and what not when this is meant to be done manually and shoved into GLSL as a uniform.
Are there any benefits to this approach?
There is a legitimate reason to do this, in terms of user sanity. Fixed-function matrices (and other fixed-function state tracked in GLSL) are global state, shared among all uniforms. If you want to change the projection matrix in every shader, you can do that by simply changing it in one place.
Doing this in GLSL without fixed function requires the use of uniform buffers. Either that, or you have to build some system that will farm state information to every shader that you want to use. The latter is perfectly doable, but a huge hassle. The former is relatively new, only introduced in 2009, and it requires DX10-class hardware.
It's much simpler to just use fixed-function and GLSL state tracking.
No benefits as far as I'm aware of (unless you consider not having to recode the functionality a benefit).
Most likely just laziness, or a lack of knowledge of the alternative method.
Essentially because those applications requires shaders to run, but programmers are too lazy/stressed to re-implement those features that are already available using OpenGL compatibility profile.
Notable features that are "difficult" to replace are the line width (greater than 1), the line stipple and separate front and back polygon mode.
Most tutorials teach deprecated OpenGL, so maybe people don't know better.
The benefit is that you are using well-known, thoroughly tested and reliable code. If it's for MS Windows or Linux proprietary drivers, written by the people who built your GPU and therefore can be assumed to know how to make it really fast.
An additional benefit for group projects is that There Is Only One Way To Do It. No arguments about whether you should be writing your own C++ matrix class and what it should be called and which operators to overload and whether the internal implementation should be a 1D or 2D arrary...
