If yeast is collected for re-pitch from secondary, the brewer is collecting the less-flocculent yeast from each batch, so is likely to end up with an issue with the beer dropping clear over a few generations. Conversely, if it's collected from primary after transfer to secondary, you're likely to end up with highly flocculent yeast that may not attenuate fully. Of course, the majority of home brewers on HBT don't use secondaries so this shouldn't be an issue (a fully completed primary ferment should have a good mix of yeast in the slurry). I'm conscious of it because I ferment in kegs and often transfer to serving keg quite early (before FG is reached) and spund in the serving keg. I'll only reuse the yeast from the ferment keg a couple of times this way so I don't get problems (or overbuild starters).
Nice. You must have very good harvesting and storage methods. It's generally not recommended to go that long (although some strains work better than others) unless using lab techniques, but if it's still working then keep going! Are you using slurry, overbuilt starters, slants, or isotonic salt storage?