Invasion sensitive is right, the point is to keep your herbalists inside during times of trouble- early on its a great time to have them help processing. After the goblinite has been harvested you can return to gathering. Particularly during summer if you fence in a handful of trees, you can easily squeeze an entire year's worth of food out in under a season. I got 4 proficient herbalists to hit ~2000 plants by the start of summer. I'll need to experiment a bit more to find roughly how much space is needed to grow that many herbs alone in a year.
Now, cage-trapping the caverns has always been ridiculous amounts of food- particularly if you use your military to help out the process. Others have made ample use of a "closet full of pigs" that continually generate bacon (and milk) from dust and wishes. Herbalism as the SOLE food source has never been that viable. Sure, quarry bushes and sweet pod food can be stretched rather thin, but you'd still need to gather enough textiles and ohter booze-able plants to cover your entire fort. Between dimple cups, sweet pods, quarry bushes, and pig tails being used for food/textiles, you'd have to gather 640 plump helmets/dwarven wheat a year (and 300 pig tails at least). This was entirely doable, but for hte most part you were using livestock/hunting/cage traps as your "main" food source and allowing herbalism to be the supplemental. The remainder (mostly cloth) was picked up from caravans.
The point of my post, though, is to highlight how broken plant gathering is atm. I've always kept an herbalist on deck, its free food just laying there! If nothing else it was always a good idea to dump the plump helmets and skimp on booze. While you have one guy chopping trees and another 1-2 mining out rooms, the rest of your four dwarves could spend at least a couple of weeks harvesting from 0 skill. Later on a legendary herbalist could bring in hundreds of plants a year... but my point is that now a non-legendary herbalist can bring in several hundred plants a season.
Before multi-hauling, they'd waste insane amounts of time picking a plant, running back to pick up a barrel, lugging that (heavy) barrel all the way across the map, and then finally bringing the plant all the way back to your stockpile. Even with feeder stockpiles, you'd still see the herbalist return to base for each harvest job. Now, I see dwarves picking 30-40+ plants in a row before returning with 100-150 easily after a couple of weeks.
From what I've gathered on hte forums, many people have just gotten into the habit of ignoring herbalism entirely. At most they'd turn their 5th legendary cheesemaker into an herbalist for something to do, but mostly I've just kept coming across people that don't even consider it. Not at the beginning when your dorfs are just milling around the wagon, not at the end when you have 20 free dorfs doing nothing in your dining room. I keep reading over and over about how you need to rush and get that first farm set up for a spring harvest... how many farming plots do you need to work to feed your dorfs, etc. Most of it has just revovled around farming as the main food source. Every now and then you'd come across someone that set up live-stock as a main food source, but never have we been able to use HERBALISM as a main source. Its gone from being a niche (like bee-keeping) to the easiest and most powerful food source to exploit.
I just wanted to point this out for all the people that have been ignoring herbalism entirely. Even if you don't bring an herbalist you can still see huge gains sending your idle dorfs off to clear the area around the wagon for some large gains. And if you really want to be silly... If nothing else, people can safely ditch all their food/seeds/booze on embark to save ~250 points. Just butcher your pack animals and quickly harvest/brew up a batch of booze. You can bring 7-14 units of booze as a "buffer" as well. It helps to disable barrels in your first food stockpile to ensure your brewers have something to work with.
Yes though, this is redonkulous. ~5 non legendary gatherers brought in 2000 plants in under a season, without using fruit trees. By the end of summer, I had well over 4000 harvested plants. So, two seasons, 4k plants. If you have been ignoring herbalism up till now, don't! Stop being a dummy and bring one on embark. A single herbalist/cook/brewer/processor can supply your entire early fort, freeing your other dorfs to set up the infrastructure or even military.