Although I'm sure John Romero wasn't the sole advocate of demo-support in their games, his presence and excitement during their in-house "deathmatches" exemplified the need to ensure its existence. From those sessions, the rules and dynamics of modern multiplayer personal computer gaming were born. Had this developed differently at this stage, First Person Shooters as a genre would have evolved much differently.
As with Wolfenstein, the levels were timed. This meant the game was quantifiable. Demo-support meant you could compare your times with others. One must realize that in 1994, the state of the "internet" was much different than it is today. Bulletin Board Systems and Ftp sites were dominant for file storage. Can you say 9600 baud phone modem?
By the time Windows95 was released, there were hordes of DOOMers playing together in online deathmatch games. There's a famous statement that DOOM was loaded on more systems than any other piece of software1... widespread playing meant practice, practice meant competition; and the competition was intense. 20-minute one-on-one matches meant sometimes well over 100 frags; not even Quake would be this fast-paced. DOOM became the featured game of online competitions and LAN events... true PC gaming as a professional sport was born. As with Quake, the core DOOM deathmatch gameplay and dynamics would hardly change as the years progressed (outside the advent of the almighty SuperShotgun in Doom2). However, single-player would see a good amount of progression and variation.