I mean "sure" only as a manner of speaking, not to say that I am absolutely certain. The challenge for me in using the illegal opcodes is internalizing them and recognizing the situations where they can actually be used effectively to save a byte or a cycle. When writing code for platforms with very little memory or harsh time constraints, they can certainly save you headaches. Something like LAX for example can be used to do an y-indexed load to the x register, which often times can save you an accumulator transfer when doing doubly indirect loads.