This is what I do pretty much every batch. It's the healthiest yeast in my brewing process and if spare yeast is going to end up in a mason jar anyway it might as well get there without going through the beer and all the other opportunities to pick up uninvited guests.
I see home brewers make this incorrect assumption all of the time. However, nothing could be further from the truth. Unless one is performing aseptic transfers into wort that was autoclaved in the vessel in which the yeast is propagated and stored, pitching a starter from a starter provides little to no advantage from a contamination point of view over serial repitching from normal gravity (sub-6% ABV) batches. Boiled starter wort is not sterile, and neither is a sanitized or boiled starter vessel.
If the processes compared are:
1. Make a starter, unload the starter liquid with some of the yeast into a sanitized mason jar; or
2. Make a starter, unload the starter into a sanitized fermentor, use sanitized bottling equipment to remove the beer, then unload some of the trub into a sanitized mason jar;
Then I hardly see how the first is a less effective method of yeast recovery if only because less sanitized, but not sterilized, equipment is coming in contact with the beer.
I'll agree with you that if the comparison is either of the above processes versus pitching new wort directly onto some or all of the trub from a prior batch then certainly that direct pitch is a superior process. However, when I brew I am almost never brewing another batch immediately after the next and often not brewing sequentially with the same yeast. So either way that yeast is going in a mason jar. I also have to store fermentors in an area where there is a lot of dust that finds its way to the exterior of the fermentors. These are screw top one gallon jugs for the most part. It's difficult to get them effectively clean to pour out the yeast without picking up an infection. I've tried with a near 100% success rate of picking up a very aggressive infection. Going directly from starter to mason jar has solved that problem. That's why I qualified my answer that this is the best option for my brewing process rather than insist it is best for all.