Is tonearm bias a compromise, maybe a myth?

Skating is a component of the force vector generated by the friction between the stylus and the vinyl. If the stylus on a pivoting tonearm is not in line with the center of the pivot bearing, then the x and y components of the friction force try to pull the stylus out of the tonearm, ie. away from the bearing and also left (towards center) of the tonearm because the stylus is to the left of the pivot bearing. Linear tracking tonearms have no anti-skating adjustment because the stylus is in-line with the sliding arm tube bearing. It has nothing to do with the grooves. The tonearm will be pulled to the center of the record even with a blank vinyl disc. Since the counter force, or anti-skating force must be applied at the pivot of the tonearm, cartridge manufacturers apply a bias or prelaod to one side of the stylus/cantilever suspension just like they apply a preload to the suspension in the vertical direction. That is so when the cartridge is cued onto the record, the VTF load and AS load center the stylus in its suspension- as long as those loads are to design intent. Try to look at the stylus cantilever on your cartridge sometime when it is in the free state. It should look offset down and left. Then cue it onto a record and it should look centered and left. Now start the record turning and it should look centered. That is if the loads are adjusted within the range of the cartridge manufactureres' specifications.

Tony, interesting post. Regarding pre-loading and non-pivoted arms : does this mean that the cantilever will be permananently skewed when a cart is fitted on linear tracking arms?

I can't say for certain, but I think that the stylus/cantilever pulls the tonearm along, so if those loads are balanced properly then it is going to be centered. I had a buddy who used the ET2 linear arm on his VPI table. It took him a few years to figure it out, but I recall him telling me that he had to level his table a certain way- slightly downwards, I think and it would track perfectly. He also said that he had to keep the air bearing tube perfectly clean. It would sound grainy if the tube had any dust on it.
Also, cartridges are different. I think some may have a different compliance in the horizontal direction than in the vertical direction, ie. the cantilever may be stiffer in the horizontal direction. So you may not see as much deflection horizontally on some cartridges. My Benz cartridge has what looks like an o-ring on the cantilever for a suspension. So the bias downward and slightly to one side is very apparent. But when riding on the vinyl it is centered. The cantilever goes through a hole in the housing which makes a good reference for seeing the cantilever position.
When I am setting up my tonearm and setting the HTA, I have the protractor on top of a record so that the arm is at the right height. Anti-skating is set to zero. I pull on the platter while viewing the protractor alignment. This applies friction drag to the stylus and sets the cantilever in the "as playing" condition. I can see the difference in the protractor alignment when turning the record slowly versus the record remaining stationary. (The record slides along under the protractor).

I just uploaded a couple of pictures of my phono cartridge for illustration on my
system page. One shot shows the cantilever in the free state and resting low
and to the right. btw- I said that wrong in my previous post about being down
and to the left. (I'm left handed. We get our rights and lefts mixed sometimes.)
The other shot shows the cantilever cued onto the record. I tried to get a good
angle. It shows the cantilever very nearly centered. The angle of the camera
shows it off a little, I think, but in reality it looks centered.
I usually end up at the high end of the VTF tolerance from the manufacturer and
my AS force set a little less than my VTF. I also keep the stylus clean with a
wetted stylus brush. That is key to maintaining good tracking.
I have a 45 rpm direct to disc record of the Appassionata that is very
challenging for any tonearm/cartridge. It is very dynamic. Once my old Koetsu
was worn I had to stop playing it. It could not track the record on the
crescendos. This cartridge tracks it perfectly from beginning to end and the
peaks do not come until the end. It gives me goosebumps.

Tony...as a fellow left-hander, I sympathise :)

You must have nerves of steel m8! ("DJ scratching" - in the "safe" direction - on your protractor :) :)
I'm usually a quivering wreck just making routine adjustments :)

It reminds me of NSGArch's repeated appeals that platters be secured during cart alignment to avoid any accidental cantilever reversals. Sadly, few of us - including myself - take this simple precaution (I will next time, I promise!) :)
On my turntable, I've noticed that after deceleration the platter comes to rest with a final small oscillation (backwards-forwards, about 1mm or so, for a split second). Perhaps wrongly, I attribute this to the rubber belt stretching alternately either side as it grapples with the heavy platter's inertia...
It doesn't encourage the idea of not being present during a power failure while a record is spinning! :)