Detachments, especially the close combat detachments, are what takes Empire infantry from cheap bodies that can potentially hold a line to an infantry group capable of winning combats convincingly and breaking their opponents. Without the flanking detachments, most enemy units will be able to stand toe-to-toe with the Empire infantry, and many of those will actually be able to break your infantry blocks with decent regularity. With the detachments however, Empire infantry blocks become much more difficult to defeat, let alone break, thanks to the flank bonus, rank negation, and extra attacks.
In short, yes, Empire detachments are always worth taking, how many and of what kind though varies depending upon what is in your army and the kinds of tactics you wish to employ. By and large though I think every combat block of infantry should also have at least one supporting detachment of combat infantry to counter/support charge. They may not be necessary per se, but they definitely improve the effectiveness of your army in any given situation. Besides, having cheap units that can cancel ranks, claim objectives and do not cause panic in other units are always worth having.
Tyler says: You are not your avatar. You are not your special title, your rules knowledge or your last article. You are not your signature... You are not your post count.