\version "2.10.33"

\paper{ ragged-right=##t }

\score {
  {
    \time 2/2 
    \relative c' {
      c4  d8 e f g a b |
      c8 d16 c \set tupletSpannerDuration = #(ly:make-moment 1 4) \times 2/3 { b8 c a  g a f  e f d } |
      c1
    }
  }
}

\markup {
  The result is as expected. 
}

\markup {
 But I would prefer the following default beaming,
}

\markup {
  without being forced to manually enforce it, even in Alla Breve:
}


\score {
  {
    \time 2/2 
    \relative c' {
      c4  d8 e f g a b |
      c8 d16 c \set tupletSpannerDuration = #(ly:make-moment 1 4) \times 2/3 { b8[ c a]  g[ a f]  e[ f d] } |
      c1
    }
  }
}

\markup {
 By the way: Why do the brackets disappear? Is this behaviour intented?
}