The reason that styrene with a metal core splits on a bend when styrene cement is used is because although the metal core prevents the tube from kinking, it will not prevent the plastic tubing from stretching (and therefore thinning) where it is bent. As the wall of the plastic tube becomes much thinner as a result the application of styrene cement will go straight through it causing the plastic to split.
An earlier poster mentioned that the inside diameter of plastic tube can vary. I disagree with that as the tube is made by the extrusion process through very high precision tooling.
Daniel mentioned using Plastruct's non polystyrene tubing that he thought to be ABS. it is. Incidentslly, normal polystyrene cement does not work well with ABS. Plastruct's own Plastic Weld liquid cement works better on this stuff. In fact it works on a variety of different plastics and I tend to use it as an all rounder on my models.