Basically a good mesmer can get out of any tricky situation using his clones and phantasms by shattering or using them normally.
This. Situational awareness is a key aspect of a good player(regardless of profession) How can I tell a good player? Simple, they skill and gear for the situation. Think of the team and ways to keep them alive as well as kill stuff fast.
There are some optimum loadouts for sure, but that doesn't negate the usefulness of others in certain situations. A player who always uses the same skill, gear and traits for every situation will do ok in the game, might even be seen as an amazing player for the amount of damage etc they can do. But for me, a player who knows the tools in their toolbox, thinks ahead and builds and plays for the team will always be a more complete one.