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.
Additionally, let's compare the vitality of yeast cropped from a 1.050 batch of wort that was pitched with the cells from a 1L starter and yeast cropped from pitching a 1.5L starter with the yeast cropped from 500ml taken from a 1.5L starter. The maximum cell density (a.k.a. saturation point) for 1L of wort is approximately 200 billion cells (the value is approximate due to the varying size of yeast cells). If we crop 500ml from a 1.5L saturated starter, we are starting with roughly 100 billion yeast cells. When pitched into 1.5L of starter of wort, 100 billion cells has to double log(300 / 100) / log(2) = 1.6 times to reach saturation. Now, let's compare and contrast that figure with pitching 200 billion cells into 5 gallons of wort. Five gallons of wort is 19L; hence, maximum cell density is 19 x 200 billion = 3.8 trillion cells. With a maximum cell density of 3.8 trilllion cells, 200 billion cells has to double log(3,800 / 200) / log(2) = 4.24 times. Guess what happens if we serially repitch a starter versus serially repitching yeast cropped from a normal gravity beer? If you guessed that the yeast culture loses vitality more rapidly, then you are correct.
Now, I know that some of you are saying to yourselves, "Mark must be delusional. We routinely ferment batches with 1L starters that were pitched with 100 billion cells from a White Labs vial, and the batch ferments well." Well, the difference is that the 100 billion cells from the White Labs vial came from a yeast propagator that was inoculated with a small seed culture; hence, the cells in a White Labs vial are mostly young cells. Therein, lies the difference between cropped yeast of any kind of lab grown yeast.