I'd say not having one would make a weaker leader.
Today the degrees have been commoditized, and without MBA one may find himself in a disadvantageous position. The new age CEOs or the seasoned ones, need to supplement their on-the-job experience with formal education (now that sounds like essay material
I have seen executives who don't have an MBA but are top of the line leaders and then on the other end of spectrum are highly decorated managers who don't seem to bring much value to the table (at least commensurate to their compensation). The former league is a rare find and is that of born leaders with a knack and talent for it. The majority of successful ones I've come across (in the middle of the spectrum) are having a strong foundation in the form of a MBA... and I guess companies have recognized this too. One consulting and two financial firms I was with in past built their own B-Schools, in collaboration with schools such as INSEAD, HBS etc., and with their own customized programs.
Recently I've been reading the Daily Drucker (Peter F. Drucker) and whatever doubts I had of falling into huge debts to get a MBA in current market is dispelled by this huge proponent of formal managerial education.