Well by that definition, anyone who is not an MLB regular already should not be given a chance because they haven't "proven anything".
And its not like Pompey failed miserably or anything, his 88 MLB wRC+ over 150 PA's is identical to Pillar's career 87 wRC+. Steamer projects him to be a below average bat (92 wRC+) but there's reasons to believe he'll be a positive contributor with his defence, more than two useless f***s we have right now taking up 2 roster spots and between them don't have a single skill that we can consistently rely on.