Battlecruiser are good for attck or defense???  is it worth to include them to battle group fleet???

they are good for both! best way to check is look at there specialization to see what they damage and what they take damage from

Wrong question. I guess u wanna know how good is "Battlecruiser" against other units. To answer this question u should check it's specialization.

"Battlecruiser" has att bonuses against medium size units (x1.5) and any unit with strengthened armor (x3).   

Let's descry "Cruiser".

Here we see it's a medium size unit with strengthened armor that's mean our "Battlecruiser" has x4.5 bonus att against "Cruiser". 

Hope I answered ur question.

P.S. "Battlecruiser" has best synergy with "Destroyer". 

Thank you to all.

exelent answer and explanation  couldnt be better or clear thanks to Exterminator