The pop system isn't completely optimized, yeah. But the xenophile ethics still has several advantages:
- it's easier to control who you enemies are, and more generally, defensive pacts still work as a way to prevent wars (AI empires won't try to attack a stronger alliance).
- the xenophile faction is both easy to please, and tends to grow exponentially as you start getting alien pops. That means lots of influence.
- Xenophile make migration treaties easier to get. While it currently doesn't benefit that much to your economy (since only one pop will grow at a time), it can still provide pops who can colonize different planet types.
- wars of conquest will go smoothly, with less time needed to absorb new pops (great combo with spiritualist and governing ethics attraction bonus)