Allocator Part of Type

[jmh530]

I recall some talk Andrei did where he said it was a bad idea to 
make the allocator part of the type.  However, the container 
library in dlang-community(says it is backed with 
std.experimental.allocator) contains allocator as part of the 
type. Automem does too. Honestly, I would think you would really 
be hobbled if you didn't. For instance, if you want to expand a 
DynamicArray using the built-in ~= operator, then you really need 
to know what the allocator is. Without ~= you'd have to rely on 
functions (maybe member functions, maybe not).

So I suppose I'm wondering why is it a bad thing to include the 
allocator as part of the type and why is it that it seems like in 
practice that's how it is done anyway.