Assignment in ternary condition operator

[JN via Digitalmars-d-learn]

int a, b;

if (a = 3) { }   <- not allowed: Error: assignment cannot be used 
as a condition, perhaps == was meant?

b = a = 3 ? 4 : 5   <- allowed


I believe the second case should be disallowed also. It seems 
illogical, that the first one isn't allowed, but the second one 
is, when the second one is also 'assignment used as condition'. 
Is there a valid usecase for such assignment?