Why did we go under in formation? Because we were in formation at the end of the field when posed with the choice. We did a lot of our spraying in formation. This allowed precise spacing when working a field with several airplanes, eliminated skips between passes, and cut down on the number of flags needed for any given pass. This reduced cost and produced a better finished product (a field with few or no skips).
We also did the formation initially because they figured it was the safest place to stick a new snot nosed kid like me; sandwitched between two experierienced pilots where I couldn't hurt anything.
As far as getting into ag, the process hasn't changed, much. Generally one needs to find an operator who is willing to take you on. Go to work loading chemical and doing other chores for the operator, and over the next few seasons, you'll get a chance to start working into a seat by getting an hour or two here and there putting out rinse water at higher altitudes.
Generally folks will attend an ag school to get a rough introduction to what it's all about, though not always. Some good hands-on experience doing conventional gear and contact flying is a real plus.
The operators to look for are ones who have a lot of acerage to handle, and carry an extra light ag airplane such as a pawnee or brave. Generally you'll get to spray out 50 gallons of rinse here and there, and you'll eventually be worked into a position where the operator can use you to do pesticide work, and eventually, herbicide work. If you can get into a position doing seeding or fertilizer work, so much the better; you'll fly more, and fly higher, and you don't need so much experience. The loads are a lot lighter, and you won't be turning so much or loading the airplane nearly as much as flying chemical.
Regardless of your turbine background, plan on flying piston for quite a while. It's largely an insurance thing, plus the fact that there are a lot more piston airplanes than turbine. Most turbine seats are taken by more experienced ag pilots who tend to keep those positions from year to year.
You're right that the number of positions are drying up, as is much of the work. This year was probably one of the worst that ag operators in general have ever seen. The business doesn't make you rich anyway; since the late 40's the business has increased from around a buck an acre to four bucks an acre (plus gallonage and chemical). You won't see that small an increase in hardly anything else...expenses have increased a lot more, but operators are restricted in what they can charge by what the farmer is willing, or able to pay.
The advent of larger capacity turbine ag aircraft has meant fewer jobs, and higher requirements for the jobs. That means less starting positions, and that existing positions have much higher competitive demands with respect to pilot background. Widespread droughts have hurt a lot. The whole political scene, with flight restrictions and the public panics over ag airplanes and white dust, hurt a lot of operators and put a number right out of business.
Conversely, once you do have the experience, it's a smaller field to search in. That is, qualified and experienced ag pilots aren't as plentiful as pilots in most other types of flying. Accordingly, sometimes it may be easier to find work than if one's area is corporate, or airline, etc. Or a lot harder, because the number of opportunities are a lot less.
The real crunch is the seasonality of the work; it's hard to hold down a regular job in other areas of flying, or doing much else at all, because your year will be so cut-up. Some guys will find year-round rice seats and hold them, others will jump from job to job, doing forestry work in the southeast, wheat in Kansas, sugar beets up north, and then catching boll wevil or other work as it pops up. For someone starting out, it's a matter of finding what's left. If you're interested, you can get a taste of what it's about by looking into an ag school (though it isn't absolutely necessary; operators are still out there who will train you over time...but be prepared to be very patient, do a lot of dog work, and get paid very little).
Good luck!
