Why do egg white cartons warn that pasteurized egg whites shouldn't be used for whipping or meringue?
Egg white foams are a delicate thing.  They are easy to do, but also easy to mess up, and many things will do so. Here (Wayback Machine) is a blog that addresses many of the common or less so do's and don'ts of foams and tests them to see if the writer agrees with them effecting things.  Items addressed are should eggs be new or old, at room temp, when to add sugar, and if pasteurized eggs work.  For them, pasteurized was fine.

The general idea behind not using pasteurized as I understand it:  For quality foams, the proteins in the egg white must unfold correctly during beating.  In the process of heating the egg for pasteurization, the protein may well be chemically or physically altered and the ability to unfold it correctly for a foam might be reduced.  In their test, it worked fine, just as it seems to have for you.  But, at best, it may not be as consistent or forgiving.  If you are slightly off on another aspect, say whipping speed, it may increase the odds of failure.  Also, you may have gotten a batch of eggs that were pasteurized at an optimum temp, while another batch might have been done at a few degrees higher and affected the protein strands more and that batch might fail. I personally would consider it one more factor that might increase failure rates, might increase the need to add cream of tartar or lemon, might fail with sugar, etc.

Just opinion, but if it works for you, go for it, but expect that maybe some times it won't.