I did exactly this thing when I was a broke teenager. The files in my ID1 directory that I shuffle around from computer to computer to this day came from that disc 30 years ago. (I still have the disc, but not the case.) I did, however, purchase Quake II and III when they were released. And many years later bought Quake on Steam. I think they got their money's worth from me after all.
(There were some who believed that making the shareware disc easily-crackable was an intentional stroke of genius. It got people who couldn't afford the full retail version to buy the game and expand its popularity. The argument being that $10 was a fairly ludicrous amount of money to pay for a single shareware game. Shareware CDs tended to be $5 on the high end, or free with most computer/gaming magazines.)
(There were some who believed that making the shareware disc easily-crackable was an intentional stroke of genius. It got people who couldn't afford the full retail version to buy the game and expand its popularity. The argument being that $10 was a fairly ludicrous amount of money to pay for a single shareware game. Shareware CDs tended to be $5 on the high end, or free with most computer/gaming magazines.)