According to Goedjn :
Air entrainment (and being careful with making sure you have proper mixing/good quality materials) is the primary cure for spalling. Fibermesh is supposed to prevent cracking like rebar does. However, some suppliers suggest/say that fibermesh helps limit spalling, early cracking, resistance to chipping and improves surface durability too.
Obviously the suppliers have an incentive to tout the benefits, but the existance of industry (eg: ASTM) tests to show the actual improvement shows there must be some truth in it.
I have no doubt that steel mesh or full rebar (eg: stress concrete) is better than fibermesh in terms of brute strength, but fibermesh is very cheap compared to rebar, and is a good choice in situations where you want a little more reassurance with situation that doesn't really need steel.
I'll be doing something similar when I experiment with making some "hypertufa". The recipe calls for 2 parts portland cement, three parts sifted peat and three parts perlite. And a "handful" of concrete fibermesh (a cup loosely packed for 5 gal of dry ingrediants).
[Making some synthetic rocks to stick on top of cedar log fence posts.]