Its there for crash protection. Not for chassis stiffening. Its up to you whether or not you want the crash protection. also, if you are in an injury related accident, your insurance may not be too happy if they find out a critical crash protection element was removed and may have contributed to more severe injury. Something to think about.
From a Bigsky post:
While the subframe is most generally thought of as helping w/ offset collisions, there are some that think it also helps adds some rigidity. Gary S ran his USTCC WRX w/ the subframe as he thought it helped prevent some unwanted fore/aft movement of the front suspension.
There are some lighter weight substitutes out there, my guess is they preserve (some designs probably add) any rigidity the oe subframe may add, but hard to guess if they add any usefulness in helping w/ collisions.
Interestingly the jdm v7 RA came sans subframe (weight savings I'm sure), but the v8+ spec C's have since all retained the subframe- this MIGHT speak to some performance improvement the subframe adds.
Also while losing any weight is helpful w/ our rather portly Imprezas, this weight is very low and is neither rotational or unsprung, so on a diet scale not on the most desirable weight end of the scale to shed.
Here is 11 pages of reading Who is running without their subframe? - Brakes, Steering & Suspension - NASIOC