They actually kept their failures as specimens? Hhmmm, interesting. I would expect such embarrassments to be eradicated. The Admech and/or Inquisition delegation doing a geneseed tithe inspection would be really interested in seeing said specimens. "What are these...things? Failed aspirants you say? And just how many percent of aspirants manifest this kind of mutation? 6 percent!? Are you serious? This won´t do. This really won´t do."
The Ian Watson novels were very early Black Library works, and the general perception seems to be that the "Canon" has moved on a bit since that time. (Though personally I think all 40k material is equally valid and capable of neat reconciliation, if done carefully.) So yeah, that image does seem a little out of step with "modern" interpretations of the Marines and the regular Inquisitorial nosings-around their Apothecaria.
That said, modern doctors keep samples of aberrant tissue etc, so perhaps those chapters who are better at keeping their geneseed stable are the ones who face up to the risk of mutation and study it carefully? Those who have a more medieval mindset ("Burn the impure flesh!") might actually regard mutation as a spiritual impurity and burn it, rather than study it, thus damning their chapter's geneseed to further degeneration.
Although of course 40k's tendency towards accepting many viewpoints may also allow for such a medieval mindset to be totally correct!