1. There is not, from my perspective. The relay policy for OP_RETURN takes minimal effort to maintain and has no risks.
2. Lazy developers that want to store less than 1KB of data on-chain. With inscriptions, they need 2 TX, while now they can do 1 TX.
Otherwise, large inscriptions still are 4x cheaper if done with SegWit instead of OP_RETURN, so no one will stop doing that for jpegs.
3. There are none so far I am aware of that do not involve the sole purpose of data storage or tokens.
Current use cases such as OpenTimestamps, Silent Payments, etc can all operate within 80 bytes.