Modern CGI is breathtakingly complex and is a much needed tool nowadays in most blockbuster films.
In Aliens, the suit cannot be left, and you cannot even think that they would throw away the suit.
It's like Godzilla.
While people want CGI in the new films when they get made again, they do not want the suit to never be used.
It'd be like if you used CGI for a fight in AvP.
You can have much for freedom to make a cooler battle than just having two guys wearing rubber suits roll around with each other.
In Hellboy, both Hellboy and Samael were suits, yet when they had close fighting it switched to CGI for a better effect and to make it more believable.
As for Aliens, I can see CGI used to create Hordes, and to have them doing things a man in a suit cannot do, such as climb on walls and scurry around quickly, but all close up shots will be in suit.
