Fleetwood Mac may have released seventeen albums, but nine of these came out before Lindsey and Stevie joined the band, by all means check for yourself:
http://en.wikipedia.org/wiki/Fleetwood_Mac_discography
Thus Fleetwood Mac have over this time released eight studio albums - just one more than the Eagles. In order the eight albums are:
Fleetwood Mac (''The White Album'') - 1975
Rumours - 1977
Tusk - 1979
Mirage - 1982
Tango In The Night - 1987
Behind The Mask - 1990
Time - 1995
Say You Will - 2003
From this, I would say that they may be able to knock an album out in a couple of months, but they've not seen fit to release many since the 1980s.
Frankly I think Time can be discounted from this list - according to the credits it does not feature Stevie Nicks at all, and features Lindsey Buckingham singing backing vocals (not lead vocals, and no guitar) on one track. As someone who loves Rumours dearly, I have never been remotely curious about listening to this album. Would you accept an album featuring - for the sake of argument - Bernie, Randy and Joe, with Glenn singing on the harmonies on one track and Don nowhere to be seen, as a proper Eagles album? I know I wouldn't.
Also, there have been times when Fleetwood Mac's lack of a unified creative vision has harmed their music - Tusk is IMO the best example of this. Lindsey Buckingham's minimalist production style suited his own material (with strong influences from punk and new wave, although also bluegrass to a certain extent) but I think was totally unsuited to Christine's songs, bar the well produced and classic Mac sounding Think About Me. Her songs on that album for the most part were not especially strong pieces of writing and needed a fuller sound to highlight the melodies, whereas instead the stripped-back production highlighted their bland lyrics and lack of hooks.
One thing I would add to Soda's point is that Mick might be able to fire one of the vocalists, but if he did so he'd be seen as being on a par with Mike Love - a man firing the true creative talents in his band for the sake of wielding his own power. Suffice to say I have very little respect for Mike Love and although I am not a massive Beach Boys fan I think he cannot hold a candle to Brian Wilson and 'Mike Love's Beach Boys' are an empty, pale imitation of the real band.