Programmers traditionally put their "use" statements near the top of a package. If they do that and put the BEGIN near the bottom, I'm back where I started. Having an arbitrary convention to handle this problem breaks as soon as someone forgets/ignores the convention.

package Trait::Foo;

use strict;
use warnings;

use Some::Other::Module::Which::Exports::Stuff;

sub foo { ... }
sub bar { ... }

BEGIN {
  # S:O:M:W:E:S's exported stuff will get picked up
  # here.  That's bad.
}
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Otherwise, you're saying subs first, then BEGIN blocks and then other "use" statements. That basically means telling trait authors to write their traits upside down.