https://www.bigsoccer.com/forum/showthread.php?t=145596 Vote in the poll above to have your say about Seattle, at least.
Check out the field views MLS fans would have at Qwest Field in this interactive seating chart: http://seattletimes.nwsource.com/sports/seahawks/stadium/seating/seating_chart.html
There seems like there is NO chance whatsoever of a future SSS being built in Seattle. Qwest stadium is a great TEMPORARY stadium until a proper SSS is built. In Salt Lake, suburbs are already fiercely competing for a stadium deal and the team HASN'T EVEN STARTED PLAYING YET. Now THAT'S progress! In a year or two RSL will be out of Rice-Ecceles. I seriously doubt that will be a similar situation in Seattle. So to answer your question....with expansion teams, yes. SSS's are a deal-breaker.