Do Simpletests require t()?
I know for a fact (though it is probably not the only reason) that t() allows for replacement arguments:

echo t('my %string', ('%string' =&gt; 'Foo'));


Which renders as: 'my Foo' (emphasized text). 

If you were to use such placeholders on your site, they will not work unless you wrap them through t() or format_string() (where t() is much much shorter). 

Here comes the opinion part; if you only were to use plain text strings it probably wouldn't matter. But if you use t() on the front end with formatting and you would compare that value in the backend with a plain text string, it wouldn't match. And writing all tests to use format_string() instead would be very confusing.

In the end it all depends on your situation and your tests.