tonearms, straight and curvy

Which straight tonearms do you mean? A linear tracking type? 

Apart from the linear tracking type tonearms i know only one straight arm (Viv Lab) for high-end market. 

The rest of the arms with straight armtube have an offset angle on the headshell anyway. 

No difference for the cartridge. Straight or S shaped - the arm connects the pivot point to the cartridge. The length of the arm or distance of cartridge to pivot point is a bigger factor. The S shape may have an advantage in slightly reduced LF vibration - the bends at 1/5 and 2/3  of length will reduce low frequency vibration along the entire length - a kind of filter if you will. 

So the "Technics" curve allows for a "straight-on" attachment of the headshell, whereas straight arms require the headshell offset?  Do all arms with a collet attached/removeable headshell need to be S-shaped, in order to accommodate that feature?  

Ideally, you want headhsell offset; the S-curve provides the offset so that the headshell can remain straight. Make sense? Arms that are not S-shaped will need headshells that incorporate the offset at the collet.