Oh come on, beings capable of wiping out all evolved life in the galaxy every 50,000 years would surely be above petty things like attacking earth (when they could probably blow it out of space). I dunno its just i pictured it more to be a race against time while the Reapers approached with the attacks happening as a climax. Them attacking Earth just to **** off shephard seems a little stupid when you're talking about a race of giant super advanced machines.
I agree it does seem a bit silly for them to attack Earth alone, maybe they just want Shepard dead?
From what that Game Informer preview said Shepard is on Earth when they attack. So perhaps they see him as a threat or just a major nuisance who could prolong their attack and want to wipe him out fast.
He could be someone to unify the races for a bigger defence against the reapers, except the Batarians I guess