You should remain focused on what you do best.
Have a broad approach on different markets/requirements from your clients (current and to come) and same time have few employees specialized in the most demanded fields.
As a general rule, a professional which provide results in a mixed environment is to be preferred to one which excels in just one field. Is about adaptability and the ability to get the things done in the way to produce results.
There is another issue: could happen to design a campaign which you know for sure that will produce results, but your client to dislike it. If the client is big is difficult to convince about your opinion.
Bottom line, work on a wide spectrum and have few collaborators specialized in niche markets - even just for ideas and insights.
Another useful approach is to design several drafts for a campaign, which to be as simple as possible, only to show the direction and reaction of your client, and to refine that one which have the biggest chances to balance your gut and clients desire.
If you need more let me know by email at bogdins at gmail dot com or by skype at technobogdins.