At this point most new instrument acquisitions represent a small, incremental addition to an already-large toolkit. I might sit on a Kontakt library for months before finding a place for it, and even then it might make only a minor contribution.
There are exceptions, of course. Talking about those instruments that fill a major gap in your repertoire or make difficult parts easier, and thus alter the way you do things. For example, Kirk Hunter Concert Strings was my first serious string library. It changed the way I approach orchestration, made me more attentive to detail, and spurred me to tackle more ambitious string arrangements.