why press fit crankshaft?

"Compact and light", exactly.  Not something auto engines are as concerned with.

Thirdly as history has shown in the Rotax engines it works very well and is very reliable

Hi Nicola

Most motorcycles do press fit cranks and the reason is the same.  it reduces weight and makes the engine shorter (light) for best power to weight ratio.  The crankshafts are actually very reliable on this design and even the type 915 engine, with 141HP on takeoff power uses this system.  The major difference on 915 is the larger lower rod bearing and rod journal for the torsional loads.  

Cheers

However, at least the Norwegian guys do some welding on the crankshaft when they lift horsepower towards and above 140. At the end of the day we cannot cheat physics.

There is a text book on pressed cranks with tables on sizes, tolerances and pressures. Basically each crank is designed for the power and torque that the engine produces, you can get away with 10-20% more but more than that you need to either design another crank ie as Rotax did with the 915 or spot weld it as is normal in racing and as the Norwegians did.

So welding a crank does not mean it is a bad design or system, it just means you are using it far beyond what the designer intended.