Plastic feels "cheap", whether it's knife handles, bodyparts on your car, the bezel on your cell phone, or the case of your laptop. Fiberglass, steel, brushed aluminum, or the like will always feel more solid and of higher quality. But that doesn't mean plastic is inferior or unsuitable. When it comes to knife handles, it's usually a matter of price and intent of usage. I like that Spyderco uses FRN handles on knives like the Delica/Endura or the Salt series. For ~$60, they make wonderful working knives, and the plastic is lightweight and durable. I wouldn't want a higher quality handle material like G10 on my Tasman Salt, because that would drive the price beyond what I would want to pay for a such a knife, considering how I'd be using it. However, if I wanted to carry a nice looking, dressy, classy knife that I'd feel proud to show someone and expect them to say "Damn, that's a nice knife", obviously I wouldn't want plastic handles. That's why I own knives like a CF Caly 3, or my BM 940D2CF and 940Ti-01, or my Kopa in cocobolo.
Plastic has its place, but some just have no use for that place, and that's OK.