yes, offchain tx's take away from mining fees. but SC's will open up a whole new avenue of risk by not only competing at the mining level for tx fees but also competing for BTC units and competing for new investment fiat that now suddenly has a whole menu of assets from which to choose from when contemplating investing into the "Bitcoin trading platform".
I don't know what you mean by "competing for BTC units" since the chains don't really "get" the BTC units, more like just babysit them (unless the SC breaks).
As far as competing for investment fiat, aside from just the tx fees which are negligible right now, this seems to also apply to Counterparty. A stock, for example, by nature involves investing in a company rather than in bitcoins (or dollars). Someone could buy ASICMiner shares for fiat or buy paper DOW stock certificates for yuan, but I don't know if that's a bad thing for Bitcoin or the dollar, respectively. Sure the money doesn't go into the ledger, but eventually just having a stock market running on the ledger, at bottom, should bring a lot of value to that ledger.