Here's my thought.

Suggesting #!srfi-105 marker doesn't mean you should mark every file
that uses srfi-105 with the marker.  Some implementations may support
srfi-105 by default, and interpret {} as curly-infix notation without
the marker.  If you *know* you use such implementations then you don't
need to bother to mark up your code by #!srfi-105 at all.

However, some other implementations may need to switch interpretation
of {} file-by-file basis.  Such implementation would add its own
extension to the switch.  Gauche will for sure, and for the backward
compatibility, srfi-105 mode won't be the default, at least for
a while (because I know some production code relies on the current
interpretation of {} in Gauche).

So, isn't it better that such implementations use the same marker,
instead of having different markers?  Then, if you wish your code
to be extremely portable out-of-box, you add #!srfi-105 marker for
extra safety.  Of course, you can still distribute your code without
the marker and just say "my code using srfi-105, please turn on the
extension if you run it on an implementation that doesn't support
it by default."

If srfi-105 doesn't suggest the marker at all, the latter would be
the only choice.  I think it is better to have another option.