We have never bought an EP and have also never not been able to get into a house. IMHO being there for S&S is more important than EPs because you can be through 3 houses before the regular crowd is even admitted to the park. Also watch for traffic tips from people like Metro and Inkognito who are locals, they will be able to tell you which house exits to the entrance of another house. Just knowing that can save you a lot of walking from one place to the next so you can move efficiently from line to line.
If I had only 1 night and it was projected to be a crowded one I might spring for an EP. Frequent Fear passholders with S&S have the luxury of hitting 3 or 4 houses before there is any crowd then enjoying the shows and fear zones while the mobs queue up. Next visit you meander through the houses you missed. Third visit you revisit the ones you enjoyed, etc., etc., etc.
Plus, the best benefit of all with S&S is that you get to meet the people who you have been hanging out with on the boards all year