Not changing a filter is simply because it is highly doubtful it has any debris in significant quantities in it. A low hour machine, the big issue with oil is simply age and exposure to the elements. A filter is usually completely filled with oil and does not have condensate issues.
In all honesty, a filter can usually last multiple oil changes and really is not a concern unless operated in harsh environments. There is a lot of surface area. Remember too, it only filters out the 'big stuff'. Any tiny particle (typically less than 1/2 of the minimum running clearance) will just pass through and not affect anything. So filters normally allow the tiny stuff to get through. After the first runtime hours, all the left over manufacturing 'sparf' has been washed out and collected by the filter. Hence the request by Kubota to do an early oil and filter change. This I stick to and if anything, would possibly shorten the number of hours to first change. Again, depending on the working conditions.
Oil filters are cheap and it is not a big deal (and certainly easy enough to do) where a filter change with every oil change is not a big deal. But it is over kill.
Oil does 'wear out' and it gets dirty. Any oil subjected to shearing forces (going through gears) does wear out, The extreme pressures applied by the gears do cut the oil molecules up like worms though a meat grinder. This is one of the areas syns shine in that they have more longer chained molecules and that enable it to last longer. The other part of oils that 'wear out' is the additives. Over time, they simply loose their effectiveness. The dirty aspect is from blowby and condensate. Dino oil too, tends to change color simply when heated up. Syn does too but it usually requires much higher temps and if it is changed by temp, you probably over heated and need to resolve that. It is not dirty, it is just not honey colored anymore. So you need to temper the desire to change just based on oil color. A fresh oil change and a long day can result in darkened oil.
For those really interested, you can send off a sample of your oil to a place like Blackstone -
http://www.blackstone-labs.com/
and really find out its' condition.
I know a lot of people who are all over the book on oil changes. Some, only do the specified intervals, irrespective of time or usage conditions. Others, change it when the honey color goes away. One of my neighbors has a BX. He got it three years ago (I think) He does not have even 40 hours yet. He was talking to me and telling me he was going to get his first service probably this spring!!!! I told him he should of had the oil changed at the twelve month mark and he stated he only had about 10 hours on it at that time. Stored in an unconditioned space, that engine has been breathing in and out humid VA air, exhaling every hot day and sucking in cool, humid night air. I would not be in the least bit surprised to find milk colored goo on the inside of his valve cover and up in all the internal recesses.
Too often is way better than not often enough. But a filter is a closed part of the oiling system and does not have a condensation problem. So on a low hour, low usage machine a filter every other year is plenty (except that first change, then you should change it). Though I do rec. oil changes every year, even if the engine is never run.
On a really old and high hour machine, oil changes and filter changes may be needed more often though all that is being done at that point is to forestall the inevitable and a rebuild is on the horizon.
A very sensitive person can 'feel' when oil is 'going off'. Transmissions begin to feel 'notchy'. Pressure relief valves 'whine' louder. Lifters can take just a quarter second longer to pump up. Oil pressures drop when hot.
Cut open an old filter (slice, no saw or grinder) and spread out the pleats. I do this from time to time and usually see nothing except in race engines.
Back to your final point 'oil doesn't go bad, it simply gets dirty' then why ever change the oil, just a new filter should do the trick
Easy enough to test. Take a oil sample. Change the filter, run the engine for a half hour and take another sample and get them tested.
An oil filters life is over when particulate it has captured exceeds it flow capability and forces the internal bypass valve to open. Of course, you never want to get there. But if there is next to nothing in the filter, changing to does nothing (other than peace of mind). Ditto with air filters. Oiled foam type, are serviced once a year (the oils dry out) or when dirt is visible to the eye or a pinch leaves teh fingers feeling gritty. Paper filers, tapped out over white paper shows the level of debris. If it is a lot and it takes a vigorous effort to dislodge it, that filter is most certainly done. Same if it is quite few years old, even if never run, the paper drys out and becomes brittle. If the filter is just a couple of years old and still looks like a new one, it is fine. Feel the pleats, if they are hard, it is past its' prime.