All three millable plants can be brewed into booze instead, if that's what you need, but you can't have both: each plant can either be milled into wheat or brewed into booze. Longland flour, dwarven flour, and whip vine flour are all made at the mill from longland grass, cave wheat, and whip vines. A miller can convert a stack of N whip vines into a stack of N whip vine flour, worth 25 each. If prepared by a semi-skilled cook -- let's say he does a Fine job -- each one adds 75 to the price of each meal in a stack, and adds N to the total stack size. The total value added over (say) plump helmet is going to be 23 dorfbucks times the quality modifier.
That's up to a few hundred extra dorfbucks per meal. Millers are unskilled, and millstone quality isn't terribly important. For a handful of logs, a mechanism, and a low-grade millstone you can make a wind-powered mill that will crank out as much flour as you can throw at it. If you're making an infinite power generator anyway, go ahead and tack on a mill on the top floor of your reactor.
So a few minutes' work setting up stockpiles and product flow gets you another nice source of revenue and a contributor to your dwarves' happiness. Since my forts generally get through the first five years selling expensive drive-thru food to elves and humans, I always have a mill set up pretty quickly.
If you have a mill, you'll want to have plenty of bags around, though... and bags require either lots of leather or a farmer's workshop, a loom, and a clothier's workshop. I suppose you could split your bags into cheap and expensive variants using stockpiles, and then use all the cheap leather bags for mill bags, but the real synergy between mills and the cloth industry is this: the cloth industry produces bags but uses dye, and creates valuable cloth as a side effect; the mill industry uses up bags and produces dye, and produces valuable food as a side effect.