I read the whitepaper. SMT's are custom tokens somewhat like ERC20 tokens are on the Ethereum network. The creation and use of SMTs use bandwidth on Steem the blockchain. For that, STEEM the currency is needed. Also, SMT ICOs require STEEM to be bought. The organization who holds the ICO may sell the STEEM it gets later but, nevertheless, even in the worst case, some STEEM will always be held at any given time.