If you put your money into something, that money is tied up.
GM's Subaru stake was a useless investment. Roger Smith thought that he could get robots to make cars for GM in the 1980's. Along those same lines, Rick Wagoner went buck wild in the 1990's and bought up a whole bunch of stakes in automakers like Fiat, Subaru, Isuzu, etc. thinking that GM could have other companies build cars for it, so that it's CEO's could play golf all day. Needless to say, it did not work.
Subaru's technology was incompatible with GM's. Something having to do with wierd engine arrangements. After several years of nothing, they collaborated on one project, the Saab 9-2X, which was a mega flop.
Basically GM needs to free up some cash and use that money to invest in GM's own divisions. That will be the only way GM can redeem itself.