Weapon slaves are 20 times as expensive as salvage modules. None of them increase or decrease the number of blocks gathered/module/second. Whether to use them or not really does come down to if you care about the resource cost.
There are a couple of reasons to consider using weapon slaves for salvage arrays. Using a cannon slave allows you to build a single beam emitter array that simply does not stop. Hold down your mouse button and mine away. It is a much simpler way to mine than having to constantly alternate between two beams, or even worse, waiting till a single array recycles so you can start mining again.
Using pulse lets you use a much smaller number of blocks to build a mining array (typically two such arrays so you can alternate between them), which can be useful if you are adding such an array to a warship or battleminer that has more than enough power for such an array when it is not otherwise in combat, such as when it is mining. Using regular salvage modules or a salvage/cannon system will result in a much larger quantity of blocks to get the same mining results. Pulse slaved salvage beams need be no more than 15 blocks long for maximum effect. Again though, such a salvage/pulse array will use a LOT more power. This is good for battleminers. This is bad for normal mining ships.
The easiest mining array to use is a salvage/cannon array. Build a waffle pattern of salvage modules of whatever width and height you deem appropriate for your ship (I like 15-20 for fast asteroid mining, larger if eating planets), and then build another waffle directly behind that one corresponding to the previously empty squares in the waffle pattern. Think of it like a checker board. One pattern is the red squares, the other pattern is the non-red squares. Make the length of each of those waffle patterns up to 91 blocks long, depending upon your intended ship size. In the empty spaces of those two waffle patterns, fill them with cannon modules. Link all of the salvage modules to a salvage computer and all of the cannon modules to a cannon computer, join the two in a master slave where the cannon is slave to the salvage and you will have a maximum strength, solid beam that will last for as long as you hold down the fire button. Such a salvage array will eat asteroids whole in mere seconds. Make sure you have enough power to sustain fire and enough thrust to maneuver easily among asteroids.
I actually prefer to build battleminers for mining that can easily destroy pirates and pirate stations on their own. This makes mining a FAR less risky game. Because I build such ships for maximum power, I usually use pulse slaves with the salvage array so as to minimize their length. This not coincidentally allows me to build a shorter ship (typically quite stubby), which results in a much more agile turning radius. This makes maneuvering inside an asteroid field a lot less painful. Such an array can be built in the same way as above but with two of them side by side. What I do however is to instead build both of the original waffle patterns and link all of them to one salvage computer, and then build a second such waffle pattern of salvage modules 'inside' the first one, except this set is linked to the other salvage computer. Once done, this looks like a giant solid block of salvage modules, 30 blocks long and whatever height and width I have decided upon. It is not however simply a giant set of salvage blocks, the blocks have been linked to two separate salvage computers using that waffle pattern.
This last is more tricky to build (a little bit) but results in both salvage beams emitting from the same point in your ship. This is somewhat more aesthetic and makes it slightly easier to use an alternating beam, as it is in effect the same beam, just one that requires you to use two different triggers. The pulse slaves can be put anywhere you want in your ship in whatever shape or shapes are convenient. Remember though, I cannot stress enough how this will use a LOT of power. My last such battleminer actually had an internal docked power reactor to help handle the full power load.
A couple of battleminers. The smaller far side bulbous one is the asteroid miner. The larger one is a world eater. Please forgive the ugly as sin station they are docked to. Never quite got around to addressing that.
