He's hardly played at all in 3 of his 4 seasons, sure, but he's accomplished plenty during the time he has actually been on the field.
With 23 SB against only 5 CS, plus all the extra bases on the basepaths, he has been worth plus +4.8 BsR.
In his 968 innings as an outfielder he has posted +10 DRS and +8 FRV.
His career 254/333/433 batting line has been 14% better than league average by wRC+.
Of course, that is likely unsustainable with a 33.9 K% and .376 BABIP under the hood, but the results are the results and Mitchell has been a plus runner, fielder and hitter in MLB.
Add it all up, and in Garrett has notched 3.3 WAR in only 443 plate appearances, what kind of minor league free agent can you sign with that kind of upside? For comparison Perkins has 3.1 WAR in 773 PA and Collins has 2.4 WAR in 460 PA.
Can he be counted on for anything with his injury history? Very likely not. But that doesn't mean he hasn't accomplished anything.