Players without a coherent deck (symptoms include: an unnecessarily large deck, no clear goal or synergy in card choices, severe quanta balance issues, relies on an unreliable and complicated combo, etc.) often find themselves playing AI1 and AI2 because their decks can't stand up against the more structured AI3 decks.
This phenomenon seems to be more common in Kongregate than the main site, as there are usually a few people every now and then who complain (on Kong forums, Kong chat, Kong comment box, etc.) about how AI3 is "too hard."
Others like to test their weird deck ideas out on the lower AI levels.
For the last two updates (1.26 and 1.27), when Zanz rolled out a new update, the new cards aren't in the Bazaar until a few days have passed. Farming AI1 and AI5 (possibly also AI2) allowed players to obtain the new cards before their addition to the Bazaar, as AI1 and AI5 decks are randomly generated from the card pool.