My build uses Earth Spike.
I have this aspect in my Collections.
Does this aspect increase the amount of Spirit generated because it can hit multiple enemies in a line?
No, it does not generate more spirit. Spirit is generated once upon hitting at least one enemy with the skill per cast.
All Ressource generating skills (Basic skills) generate the ressource exactly once when hitting at least one enemy, if not explicitly otherwise stated (I don't know if there is any). There are many other basic skills that may damage multiple enemies easily to try that out.
You can also hit multiple enemies with the Earth Spike spell without modifications if enemies are close enough together, generating the spirit only once.
Otherwise there would be a really large gap in ressource generation between fighting tons of mobs and a single boss...
Source: Experience from playing myself