towing - dropping drive shaft?

["Geroge D. Cobb Jr." <gcobb@SPC.cc.tx.us>]

Hey list,
        I am moving some distance (550 miles) and am going to have to tow two
MG's.  I am going to move one early (verses have to move 4 cars at a
time) and was going to use a tow dolly (two front wheels off ground with
rear wheels on ground).  The rental place told me I needed to drop the
drive shaft.  My question is WHY?  Why does the drive shaft need to be
dropped verses putting the thing in neutral?  For give my ignorance if
this is a dumb question.
Thanks
Don
OfcCobb@AOL.com or gcobb@spc.cc.tx.us
1980 MGB
1977 MGB