Here are a few things regarding SMTs:

• One of the main things that SMTs will do is bring in more developers and more users. Everyone who uses a "SMT" will also be using the Steem blockchain, and all posts that are eligible for SMTs will be eligible for STEEM too. Lots of applications that are developed to support specific SMTs will work for STEEM and the other SMTs as well.
• SMTs will also bring in more capital. Users will need to buy STEEM in order to buy SMTs. This should be analogous to how needing BTC (or ETH) in order to buy a lot of other tokens from different exchanges makes BTC and ETH more valuable.
• Users who want to launch their own community and decide how the rewards for that community are distributed - without needing to get "whales" to vote, or worrying about downvotes from large stakeholders who disagree will now be able to do so (with a SMT). It will basically be a tool to form a new mini ecosystem with a new token and a new distribution.