Simon,
  I wouldn't buy one just for the modulation wheel either.  I just don't see why Yamaha leaves them off a couple models.  Of all the things they take off they choose the modultion wheel..  I don't understand why...  The PSR-740/640 have the same body molding as do the PSR-1000/2000.  Yet they cheeze you out of something as simple as a modulation wheel.  Makes no sense to me...
Squeak
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GEAR:  Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller).  SOFTWARE:  Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.