Having sold hundreds of 200-4Rs and 700-R4s, I learned over time what the capabilities of the average enthusiast are.
Most cannot install an overdrive transmission and "get it right". Many will cook the trans with an improper TV cable setup. More on that later...
Some will hook up the lockup using it as sent with 4th gear lockup. It shifts to 4th, it locks, and they are happy with that. Typically it causes a lugging condition because it shifts to OD, immediatly locks up, and puts the engine in the 1400-1800 rpm range, where our performance cams aren't super happy, the carbureted engine doesn't have great signal through the venturi, and the tuning abilities of the enthusiast leave something to be desired.
*(Some are thinking "I know what I'm doing, I know how to install/tune an engine". And SOME do. Most of them are at the track making a car go really fast. Faster than all the similar combos that occasionally show up at the track. It's not a dig on the "average enthusiast" it's just the truth. Most guys don't put the effort into their combo to get it perfectly tuned ironed out. If they did, everybody would have a wideband in their car and datalog it to get the tune perfect, percentage wise, the number of guys who do this is very low).*
So back to our lugging 450 HP typical street car.
Some guys are happy with it. Some combos work OK this way, milder engine with lots of low speed torque, deeper rear ratio, smallish carb, etc.
Some guys want it to work better (like me). They install a vacuum switch. Vacuum switch works OK. It cuts the lockup out at WOT and idle. You buy an adjustable one and get it "tuned" for best results.
Now you get a hysterisis condition that causes it to unlock going up an incline at cruise, this allows the converter to slip a bit, multiply torque and help keep the car at the desired speed.
Once the engine gains 200-400 rpm with less load, it builds vacuum, and the converter locks again. As soon as it locks, the engine sees the load and has less rpm, it loses vacuum, crosses the threshold of the switch again, and it unlocks.
Enthusiast isn't happy with it.
He's got $2k in the trans, $450 in the converter, $100 in the driveshaft, $75 in the shifter conversion or more, $200 in the crossmember, and a couple hundred more in cooler, lines, fluid, and incidentals. Now he's $50 into a switch that helps but also creates problems.
So in comes the delay valve. It helps kill the hysterisis. Usually at this point. It functions pretty well. That is IF the enthusiast had the patience to get it that far. Most don't.
Many will not buy anything "expensive" to control the lockup.
The $200 lockup control from B&M is a speed based setup that is easily adjustable and works well. Not perfect but well. It really needs to be in conjuction with a vacuum switch, but that gets pretty expensive when all combined and when you start looking at the package, it makes a standalone computer to control an electronic transmission look more appealing.
I've been on the receiving end of guys who have "ran out of talent" a few times and there IS a valid reason I recommend non-lockup in MOST (not all) retrofit applications.
As an example, some guys are running aftermarket EFI on their engines and many can output a signal based on a few parameters that will make the TCC lockup function perfect. In that case, of course keep the lockup. You can tune it to work as intended for no additional cost and no wiring/vacuum mess.