SR5 Notes about Cover, paraphrased from page 190 CRB.
Good Cover (more than 50%, +4 dice for Defense Test) and Partial Cover (25 to 50%, +2 dice for Defense Test) both benefit a Defender against Ranged Attacks. This modifier applies to your Defense Test against regular Ranged Attacks and also against incoming Indirect Combat Spells that allow a Defense Test.
However, the attacker now hits on a tie (Cover, p. 190).
Therefore statistically, Partial Cover is bad for you. I assume someone pointed this out way back in 2013 and I just missed it.