Skin effect plays no role, therefore litzewire is not necessary. In fact, in litze wire the amount of insulation can even be more, because every tiny wire is coated. This means litze wire will give a lower copper fill factor and trus higher resistance and lower efficiency.
www.flyware.de is the only manufacturer that uses litze wire, not
http://www.Kontronik.de, not Plettenberg, not
http://www.hb-motoren.de, not Cyclon, Aveox, Maxcim, Torcman, Lehner, Hacker etc.etc. It's easier to wind, that's all. About skin effect in our motors (German only):
http://www.torcman.de/peterslrk/Skin-Effekt.html
A lot of people think that be change the diameter of a stack of pipes (wires) the space between the pipes will decrease. This is not so, no matter the diameter, there will always be 10% space and 90% pipe (copper). The same goes for spheres/balls but I haven't calculated the space in between. See my drawings and my 'discussion' with Steve Neu here:
http://www.rcgroups.com/forums/showthread.php?t=65245&
It can be advantageous to wind several wires in parallel if a single wire is too thick to handle properly (and therefore gives lower a lower fill factor). From an electric and magnetic point of view there is no difference between these two types, as long a to total surface area of the single wire and the parallel wires are the same.