There are two factors which work in favour of the S-A model:

For the level of accuracy which it provides it is comparable "cheap" in terms of required computational effort.

Otherwise it depends, what researchers are interested in. With SA model you can reasonably well predict drag and lift in aerodynamics applications for small to moderate angle of attack. If you are more interested in flow separation in wing-body configurations, than the SA model would be not my primary choice and I would prefer SST model with (U)RANS or e.g. SAS/SBES models.

RANS Single-Equation Model: Spalart-Allmaras

this popularity is in large part due to the model’s robustness and fast implementation when modeling specialized flows. Spalart-Allmaras is not memory-intensive and has good convergence but it has no wall functions.The model is also a popular addition to various CFD codes.This popularity is in large part due to the model’s robustness and fast implementation when modeling specialized flows. Spalart-Allmaras is not memory-intensive and has good convergence but it has no wall functions.The model is also a popular addition to various CFD codes.

“When You look at the benefits and drawbacks, the Spalart-Allmaras model has historically been a strength … due to its speed and robustness,” .

“Because we are only solving a single equation for turbulence,” “the non-linear convergence is outstanding and the model is very forgiving of poor quality mesh, particularly in the near wall region. The drawback is that it does have some limitations due to the single-equation formulation.  The turbulence length and time scales are not as well defined as they are in other models such as SST.”

Limitations of Spalart-Allmaras include:

• Shear flows
• Under predicting separation
• Decaying turbulence

Spalart–Allmaras model is a simple one equation model and:

1- basically prepared for aerospace applications. Exactly for this reason, it is well-known for aerospace problem.

2- Another important point is that this method can resolve well, boundary layer problems subjected to adverse pressure gradients (which is very common in aerospace applications).

3- It is very cost effective from computational time viewpoint, so it is also a good candidate for large scale problems.

But it still has its own drawbacks:

1- Since some more complicated methods came into existence, it was not calibrated well for different problems.

Therefor, before using it, you should be sure if this method is applicable for your problem or not. For example it has been proved that it can not be used for free shear flows such as plane and round jet flows.

I, myself have used it for simulating flow over annular finned-tube and results was satisfactory.(Pls. refer to the following link)

Article Numerical Study of Flow Over Annular-Finned Tube Heat Exchan...