Manual boost controller is not necessary. I had one on my 08 wrx just trying out stuff and there was no real benefit to having it.
Manual boost controller:
-Easy to set desired boost level
-Easy to install
Benefits of using oem boost solenoid
-Better control for boost levels when weather changes
-If detonation is present from a bad tank of gas the ecu can reduce boost level and save the engine
-Can run different boost levels in different areas of map.
-Tuned properly can reduce lag.
My suggestion? Go with a 3 port boost solenoid. The gm boost solenoid works very well or even better is the grimmspeed boost solenoid. Very nice product and installs nicely.
The 3 port has way better control over boost than the restrictor pill bleed setup that subaru has stock and with proper tuning makes the turbo spool a lot sooner and will hold boost better into higher rpms.
A worthwhile upgrade in my opinion.