I don't agree.
Heavy stance rocks, but it's slow. It may take quite a bit of damage but is for the most part fairly easy to avoid being hit.
Many times now I've battled a heavy stancer with a yellow or blue stance successfully.
The reality is, each stance is most usefull if you used during the right situation. Heavy is good when you have space to run, and your opponent is intent on coming at you agressively. You can usually catch them with a good strike. If your overtly agressive style heavy stance can really pay off, provided yo uhave the room to manuever. Button mashers beware!!
Medium stance or yellow I found is good in most situations. It can cater to both the offensive and defensive player. It's a good stance for going in and testing the waters against your opponenet. Then switch stances as needed.
Blue or light stance I use mostly for very agressive tactics. If I see a player that is more on the defensive and running alot. Such as a heavy stance user, it can be very affective to perform hit and run attacks. You can usually get 2 hits on a heavy user mid swing if you time it right. Forcing the heavy user to either change tactics or die.
IMHO changing stances to suit your given situation is the best route. Or when your up against a player your having trouble with, complete changes of tactics can really help in winning games. The fact that once stance gets you down everytime just means you haven't experimented enough with other stances.
It can be touch to change from a defensive to agressive stance. And some players are just damn good. But as long as you keep the weaknesses of each stance in mind, and practice at quickly adapting your style then you are truly on the path to becoming a great player.