That's not what it says. It says they can't join a unit of Infiltrators if they don't have the Infiltrate rule (and vice versa). So for your example to be correct it needs to read:No, we think they're 'useless' because, as written in the FAQ, a character with Infiltrate cannot deploy with a unit unless that unit also has Infiltrate, no matter whether they use their Infiltrate deployment or not. For example, Shadowsun either deploys alone, or deploys with Stealth Suits or Kroot, because they're the only units that have Infiltrate and therefore are the only units eligible to be joined by her.
It only matters if you're Infiltrating, otherwise you're not an Infiltrator. Any other reading of the rule is going into the "your model can't shoot because it doesn't have eyes" level of silliness that troll-logic RAW revolves around.For example, Shadowsun either infiltrates alone, or infiltrates with Stealth Suits or Kroot, because they're the only units that have Infiltrate and therefore are the only units eligible to be joined by her if she's infiltrating.
That's why the rule was changed in the first place, abuse like that. The FAQ just clarifies it further since Infiltrate is a rule that normally confers as long as a single model has the rule.This reminds of Ghazghkull coming on with Blood Axes.