Does anybody know if there is a simpler alternative to playing Ikari warriors without rotary sticks?
Get really good at playing the game only shooting straight ahead?

Ba-dump-dump
Seriously though, Druin's interface makes it really easy to set up a rotary joystick. A rotary joystick has a rotary switch on the bottom, which opens and closes different switches based on where it is. The leads from these switches go to Druin's interface, a small board powered by 5 volts. It has inputs for all of the switches coming off the rotary on one side, and two button outputs on the other side (clockwise and counterclockwise). You wire your LS-30 joystick to the switch inputs, and wire the cw anc ccw switch outputs to a normal keyboard encoder. When you start turning the joystick, the interface board sees this, and then starts sending button press information to you keyboard encoder, which in turn starts turning your gun on the mame screen (turn clockwise, turn counter clockwise, etc).
It's all pretty straightforward. What is complicated are all of the options available--you can go with LS-30s, optical 49 way joysticks with Floyd's rotary hack installed, etc..
If you wanted a really simple way of playing Ikari warriors, you COULD configure a few buttons on your control panel to do the clockwise and counter clockwise motions. i.e. if you have a 6 button street fighter layout, make the first button rifle, the second button grenade, and then configure two more conveniently located buttons for turning. It would be a frustrating arrangement, but you could probably limp through a game playing that way. It probably wouldn't be enitrely unplayable.
Don't get scared off from doing a rotary setup if that's what you want. There is plenty of info online on how to set things up. The hard work has already been done for you if you buy the interface.
Good luck,
88mph