The only reason I play with a joypad is that no one else does when they're testing out ZDoom's new features and settings. I consider myself a playtester, as opposed to a gamer. ZDoom can't be perfect unless every possible avenue has been straightened out to its full potential. It's those weird gamers like you and me that have made ZDoom get to 50.cab. Just a few more things need to be ironed out, so that eventually the only person who would have a reason to complain would be Mr. Heit.
And Romero... and Carmack... and Willits...
Besides, a game like Doom (or RotT or Duke Nukem) is the only game where I NEED a "handicap," at this point. When I first bought Doom, the joypad was a matter of convenience. Now, I indulge in emulating the experience as it would have been on other consoles, as well. GBA Doom, the über-sucky and über-slow SNES Doom, even the disgraceful Jaguar and 3DO Dooms -- it's one of those weird quirks of mine. I like to take a trip down memory lane sometimes.
... but with infinite ammo, and no reloads for the super shotgun (trirailgun 9.bex). 
There are a few games where I
prefer to use the keyboard/mouse combo. These are games where precision is crucial to "playing in character." Half-Life comes to mind, among others.
Yes, I'm one of the minority in using a joypad for Doom. But hey, ALL parties need to be thought of in developing a good source port. From the "1337 h@x0rz" who like to do scripting and gameplay aliases, to the competitors who prefer to do the "
Multiplayer Marathon Spectacu-LARR"to even the oddball in the East Coast who insists on using a joypad. In order to make a well-rounded and well-balanced product, one must consider developing things that everyone else would overlook.
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