I think if we go with XML for everything else, we should not make an exception here. XLIFF is actually already specified in the standards document:

http://symfony.com/doc/current/cookbook/bundles/best_practices.html#translation-files

I don't think that being "easier" is valid, YAML is great when your in a hurry but I think its worth not compromising and investing the extra time in these bundles, but then I havn't written many XLIFF files :)