I just feel that they would give more of a thought to MAN which is its largest scheduled base afterall?
MAN is a very competitive market for leisure focused carriers - big bases for ZB, LS and EZY, and now of course FR all of whom have expanded and added aircraft and new routes in the last 18 months. Down the road at BHX, WW are about to close and other based carriers (FR and BE) only expand cautiously, with just a few new routes between them in the past two years. There is no LS and only one year round EZY route. Hence ZB have seen a huge gap in the market and jumped in and I wish them every success, finally (for now at least!) an airline that sees BHX as a strong and viable base for an expanding range of flights to a broader range of European destinations. Of course MAN will probably always remain biggest ZB base but the need to expand there right now is not perhaps as pressing (or advantageous) as at BHX.
As for destinations, BHX had no scheduled flights to Madeira or Sharm hence why ZB have stepped in this winter, I believe at MAN these services already exist. You could argue that BHX should also have flights to VRN and Friedricshafen which ZB do or will do from MAN but the business case from BHX is obviously not there at present. Of course MAN will see ZB expansion in the future as well but each base will of course have destinations served that are not available from others based on individual local catchment demand.