My understanding is that there are four main factors governing oil degradation, and hence how often it needs to be changed.
1. Contamination by other liquids, principally water (condensation) and petrol. As has already been said, a bike that's used regularly for decently long journeys shouldn't suffer from this because these contaminants should evaporate off when the engine is hot.
2. Contamination by solid particles. In theory the filter should keep this under control, but the fairly crude (in filtration terms) ones used on bikes have their limitations. In particular, on a bike, there will always be a build-up of wear particles from the clutch, which is one reason that the oil you drain out is darker than the new stuff. Personally I would always change the filter at the same time as the oil, if only to get rid of as much of the old stuff as possible.
3. "Wear" of the oil itself. Conventional multigrade mineral oils use viscosity index improvers to beef them up (i.e. reduce the natural tendency to thin out) when they get hot, These VI improvers have long-chain molecules, which are gradually chopped up by the machinations going on inside the engine (and gearbox, in the case of bikes), so the oil loses its high temperature properties and becomes less effective at normal running temperatures. Synthetic oils will last longer than conventional mineral oils because they are not reliant on these additives to the same extent. I recall that when Mobil 1 (the first widely available synthetic, I think) was introduced one of their boffins claimed that a single fill could be made to last the lifetime of an engine - provided it was properly filtered and kept free of contaminants. Unfortunately, that in effect means putting it through a centrifuge regularly, which isn't really an option on a bike. Semi-synthetics are obviously a halfway house between straight mineral oils and full synths. The fact is that semi-synths should not deteriorate significantly during a one-year/6000 mile period (unless you're really thrashing the engine, e.g. racing), so if you're doing oil changes on that basis then full synths are an unnecessary luxury.
4. Degradation of the oil due to overheating. I've never come across this in any motorcycle, though it is not unknown in things like hydraulic systems operating in hot environments. I guess if you ran an engine with low oil level, to the point of it seizing up, it could happen, but then the condition of the oil would probably be the least of your worries.
What is important is to use a motorcycle-specific oil, which should give better gearbox lubrication as well as not allowing clutch slip. Some older bikes in a low state of tune can run happily on car oil (Yamaha Diversions are an example) but I'd prefer not to chance it in a MT-09, unless there was absolutely no alternative - any oil is better than none!
