Other use cases for BEE/ENG will be added over time. For example we plan to gradually introduce more types of bot services after the market maker, which will also be powered by staked BEE/ENG.

If there are a lot of market makers running on a single market, they will compete with each other and gradually narrow the spread. However, it's likely people will have their trading configuration set differently, and orders will be placed/updated at different times in a way that should more or less be fair to everyone assuming that market volumes remain at a steady level throughout the day.