At my company stars get promoted at faster rates and are offered sponsorship for Exec - MBAs (typically MIT Sloan 1 yr program or similar) when they are around 33 - 35 (i.e. 10+ years with the company) instead of sponsorships to full-time MBAs at younger ages.
I was offered a similar treatment (the Exec MBA) but with a lot of disclaimers (I'm a borderline case as most of my fast progress has come in the last couple of years and where mostly a case of knowing the right stuff at the right resources - constrained time and company than pure stardom) when I shared my MBA plans with the company. The disclaimers:
a) I should have continued to work in the company AND should have continued to perform at above average levels (top 10%).
b) The company in no way committed to a date or time for me to attend the program, they just mentioned it was a typical career plan for most would - be high-level executives.
c) I should have been willing to rellocate as required by my career plans and options around the world at the company's requests.
In the end, I decided I didn't enjoy being in that company any longer (I did for some years and my stay served a purpose but by now I'm no longer passionate about being a part of it).
A current co-worker, who is firmly in the star track, was originally planning to go for an MBA but in the end decided to hold steady at the company as he felt the signs (i.e.: career progress, promotions, career plans and compensation) where right. Other factors that weighed his decision: the company has a firm footing in our home country and he wants to have the option to go back home whenever he feels like it, knowing that there will be enough positions available for him, and he felt that the tuition + expenses + lost income would have been too much to make the MBA worth it. I also feel that he's better suited to the company's style than I am and I feel that he's got a better chance at making senior positions (Director and above) than I had.
So that's that, for some superstars, the lost income + tuition + expenses is a strong deterrent, specially if they feel they are getting the right recognition and possibilities from their employers.
L.