Adding even just one extra surv can be pretty destabilising for the xenos on lowpop, imo. I’ve seen plenty of lowpop rounds where the survs fucked up a few T1s and T2s and ended up cucking the hive’s early strength against the marines. Adding an extra survivor to the mix is too strong.
Survivors should be extras that get wiped half the time before the main post-deployment gorefest, as they’ve always been.