How often do you fight valkyies? They arnt a common sight in the battlefield really.. and the shield bash is easily side stepped coast lots of stamina and doesn't do any damage. One thing I love to do as a valk is shield bash whoever has revenge until it's gone. But seriously I can go ten matches without seeing another Valkyrie.
This is not a problem to be blamed on ubisoft. You play with the character as you want to play with them. If you encounter someone using these cheap tactics learn how to use the spamm to your advantage and punish him for it. I've made loads of spammers rage quit out because their spam doesn't work on me.
My issue with it is that is seems to TRACK better than say the Warden's shoulder bash. The Valk can sit there endlessly in a full guard and then release with no stamina loss. Yes it can be dodged by a good player, but it isn't as easily side stepped as the Warden's shoulder bash, which makes it more effective.
I'm No spammer and play honorable every chance I can. I'm happy to annoy sucky players any day with easily dodged tricks I will also say you have to learn or lose. When I come across things that feel like exploits I won't use them. I lose all the time to someone dodging my shield rush, very risky valk can't take many hits. When you are in that situation I suggest not hungerly going for damage and try to save your own skin by rolling out. Can't hit rolling targets with that move.