not being a scientist i can only speak from my experience. i have pitched small mason jars of yeast slurry that was between 6-8 months old (after warming to room temps) and had visible signs of fermentation within the normal 12-24 hr time period expected. i have never had a contaminated beer from reusing yeast, or any other contamination for that matter. on these forums i have read about others who do the same on a regular basis. yeast washing seems like an intuitively good idea; wash all of the bad stuff out and keep the good stuff but it simply does not work that way at the homebrew level.
if i have a beer that is ready to package and is not contaminated, the yeast cake stored directly into sanitized/sterile jars has a better chance of staying contaminant free than if i washed it in the usual way.
these days i try to time my brews so that i can pitch the reused yeast within a day or two, i don't keep jars of yeast around unless they are Brett or sour blends of yeast cultivated from bottles. the reason for this is not concern for the long term viability of the yeast, i just don't have the room to store the yeast.