To your original question - why did they start going with a seperate starter as opposed to continuing to use a starter-generator like in days of 'yore ?

I'm gonna guess cost - to wind and buy a small DC motor is pretty cheap, and since it is geared down the motor torque doesnt need to be very high to get you started.
If you were doing thing right on the crankshaft, you'd need to develop a full hp or so at a few hundred rpm - In one big disk. Not only would the electrics take up more space (and they are costlier per unit volume than pieces of metal) but you'd have to effictively wind your own large-diameter DC motor - lots of factories in the far east churn out millions(?) of DC motors in any size you want - no R&D required.
and given the expected life of a lawn tractor these days (less than a car? the cost for a motor-generator is definitley prohibitive.

Starter story - a few years ago my brother brought me a 17hp 1cyl craftsman something that wouldnt start (electric start) the starter motor just didnt have the arse to turn the motor over. We tried larger batterieis and everything - finally asked my FLMPS and they said "adjust the valves, if the clearance is worn too small, the 'compression release' feature doesnt release and the starter motor isnt sized large enough to turn the engine over at full compression (!! - they cheaped out on it that hard..)
...sure enough, I showed my little brother what feeler gauges were (same guy who built/paid for a 300hp Mk2 Volkswagen...), and 5 minutes later it fired right up. The starter was that cheap/weak. blah.

__________________
2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

Husband, Father, sometimes friend =)