Article in the Lawyer here.
The short answer is No (and this from a solicitor with a MBA). Steve Jobs did not have an MBA, nor does Bill Gates, and they both built large and successful companies. And if they are too entrepreneurial, Terry Leahy of Tesco did not either.
Yes, lawyers need increasingly to think like business people, but an MBA is not necessary. Instead it is expensive, and a great deal of work. The vast majority of law firms are really small or medium-sized enterprises, and not hugely complicated. A qualification aimed largely at corporates is not a good fit.
Lawyers need to understand how their firms work, and in the present climate particularly the money side, but would generally be better off with a more tailored approach. And few people will be both good lawyers and high level managers at the same time.
On the other hand, if you want to do an MBA for fun, interest and the opportunity to meet fascinating people, go for it!