Reasoning is: anything else and a good player will slaughter you
QFT
Advent can really steamroll enemy fleets with very few losses with the right balance, but Defense frigates and crusaders are definitely a must for anti strike craft. Advent rely heavily on cap ship abilities and microing your caps, base your fleet around cap ships and build defense frigs/crusaders to support them, along with subjugators.
Note: This is the ONLY race that actually has decent cap ships... TEC and vasari usually don't bother with many caps, a kol, or a sova or something similar... not usually many of any, tho some ppl do like thier sova rushes. The reasoning behind this is all the DPS increases and aoe healing and, of course, the radiance, which just rocks all around(200+dps cap ship ftw(w/ a few upgrades))
However not balancing your fleet almost perfectly with advent spells immediate and total disaster, there is no fallback, the ship quality is pretty much all they have. No RA and no emergency LRM spam.