The fans of those organisations love those players but those fans aren't the difference between 5000 and 15000 for the Phoenix (or in City's case 1000 and 5000). That difference is the 10,000 casual sports fans who will jump on the bandwagon (or the 8000 in the Breakers case) and they couldn't care less if the jinky little winger has a kiwi passport or if he has a Venezualan one.
Gerrard doesn't bring more casual punters in he's just loved by those who support the club already and even then the UK football model is hardly comparable to NZ. If they had Lionel Messi AND Steven Gerrard I know which one the crowds would love more... the one who wins them a title and scores 50 goals a season, no matter where he's from.