Things that keep D from evolving?

[Matt Elkins via Digitalmars-d-learn]

On Friday, 12 February 2016 at 17:20:23 UTC, rsw0x wrote:
> On Friday, 12 February 2016 at 15:12:19 UTC, Steven 
> Schveighoffer wrote:
>> On 2/12/16 9:37 AM, Matt Elkins wrote:
>>> [...]
>>
>> Pass by reference and pass by value means different treatment 
>> inside the function itself, so it can't differ from call to 
>> call. It could potentially differ based on the type being 
>> passed, but I'm unaware of such an optimization, and it 
>> definitely isn't triggered specifically by 'in'. 'in' is 
>> literally replaced with 'scope const' when it is a storage 
>> class.
>>
>> -Steve
>
> note that 'in' and 'scope'(other than for delegates) parameter 
> storage class usage should be avoided.
> It really should be a warning.

Why is that?