[plt-scheme] required else for if ?

Noel Welsh wrote:
> On Sun, Sep 6, 2009 at 8:07 PM, Doug Evans<dje at sebabeach.org> wrote:
>
>   
>> How come the default behaviour of `if' in mzscheme is to
>> require the else clause,
>> and not have an optional else clause?
>>     
>
> This is one of the changes made in the v3 -> v4 transition. It is
> designed to prevent a class of bugs. The mzscheme language provides
> backwards compatibility.
>
>   

Thanks.  Can you elaborate on the "class of bugs"?

I realize mzscheme, rnrs, provide "backward" compatibility, but that 
route drags in a lot of other stuff that I don't necessarily want.