Marquette King is a bit of a head case that can cause issues but let's look at his stats versus current Cowboys punter Chris Jones:
KING (last played in 2018)
2018 20 punts 44.1 ypp avg. 39.7 ypp net 7 punts inside the 20 0 blocked 8 returns 67 yards 8.375 ypr
2017 69 punts 47.4 ypp avg. 42.7 ypp net 28 punts inside the 20 0 blocked 32 returns 205 yards 6.41 ypr
2016 81 punts 48.6 ypp avg. 41.4 ypp net 34 punts inside the 20 0 blocked 33 returns 405 yards 12.27 ypr 1 TD (note that the TD return was for 78 yards so in taking that out the stats are as follows: 32 returns 327 yards 10.22 ypr)
JONES
2019 50 punts 41.6 ypp avg 37 ypp net 18 punts inside the 20 1 blocked 18 returns 154 yards 8.56 ypr
2018 60 punts 44.5 ypp avg 39.6 ypp net 17 punts inside the 20 0 blocked 30 returns 254 yards 8.47 ypr
2017 66 punts 44.1 ypp avg 41.4 ypp net 34 punts inside the 20 0 blocked 18 returns 75 yards 4.17 ypr
After the 2017 season Dallas signed Jones to a 4-year $8.7M extension. He is set to count $2.4M against the cap in 2020 (with $1M in dead money) and $2.5M against the cap in 2021 (with $500K in dead money). If you look closely, Jones' stats have gone down after he signed his extension which seems to be par for the course with most players signed by Dallas.
Given the potential $1.4M in cap savings in 2020 and $2.5M savings in 2021 if released, I say bring in King and make it a competition for the starting punter job.