Dear Pani,
The answer to this one depends on who you speak to.
The Cartridgeman, for example, swears by completely separate platforms i.e. motor/turntable/arm pillar for the air-bearing linear arm. For good measure he even decouples the Classic cartridge with a "spacer".
Although I like the Cartridgeman’s products a great deal I cannot reconcile the operation of the tonearm without close coupling (i.e. the so called "closed loop") but it wouldn’t stop me buying them ;^)
Technically, there might still be a closed loop even for the "separatist" approach except that the final connection is by means of a shelf or a platform. Then any variability would be introduced by whatever is being used to support/level the various elements e.g. 3 solid spikes relative to whatever material the supporting surface was made of.
Provided one could guarantee the T/T & tonearm stay in precise relationship at all times I wouldn’t see a problem. Trouble is that the tonearm is by nature constantly manipulated and messed around with by the User so personally I’d feel uncomfortable if it wasn’t locked down and immovable.....
Just my opinion though.... :(