... is that you can't bind a XAML property to an arbitrary expression.
I understand the idea of separating presentation from business logic but presentation and business logic eventually do have to meet in some way. WPF as I understand it only allows this binding to happen using C# properties of a class instance that is a data context ... but C# properties are extremely blunt tools -- basically, if we forget OOP for a minute, C# properties are variables i.e. named values. Every programming language in recent times has offered various ways to access values beyond simple names precisely because simple names are often not rich enough for common use cases e.g. arrays let you access values with a name and an index, etc.