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I completely agree.
Many times ERA is more accurate than FIP or xFIP over the course of a "Career" since it catches hittablility, XBHs, the ability to hold runners, GB%, LOB% etc. xFIP is usually more accurate in the short term, but ERA isn't too bad over the course of 5+ years as long as you keep the context in perspective.
Naturally ERA is going to be biased by team defense and park (and Washburn HAS benefitted here). The park and defense aren't going anywhere though, and a SP who nukes LHs, limits BBs, gets FBs without gopheritis is going to be a TREMENDOUS fit.
Washburn is just a great bet for our current situation. We need one more SP, and Washburn at $4-5mil is a very good fit.