Because you won't know if your test suite crashes right in the middle:

use Test::More qw(no_plan);
ok(1);
   # CRASH!!
exit(0);
ok(2);
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If you run that with the harness as in PERL_DL_NONLAZY=1 perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" test.pl it will happily report

All tests successful.
Files=1, Tests=1
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because it doesn't know better. If you had specified the desired outcome, as in

use Test::More tests => 2;
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the test script would have reported

# Looks like you planned 2 tests but only ran 1.
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and the harness would have failed, allowing you to catch the bug while it goes unnoticed otherwise.