To each their own, so if you don't find them useful, totally cool. And also caveat that I'm a beginner miner too and learned a lot from the mine for dummies megathread
That said though, I've just spent ~6 hours in two long sessions of mining with prospectors and gotta say, they are really useful - if you use em right.
I'm in 160 ton cargo configured Python, 9 bin refinery, 6 collector drones and initially had 3 prospector controller but ditched it for the lowest 1 prospector controller
Even with 6 collectors, your primary limiting reagent on speeding up mining will be sitting there for at least 10-20 seconds while your drones collect all the chunks up. You really can't move or ores will be destroyed on return, so basically that's when I find it is perfect time to fire off a prospector. So what I do is:
1. thrust toward asteroid to start mining - can use a prospector or not for this first asteroid
2. after mining laser blasting and unleashing my collector drones (all 12 min rated), as I said even with 6 collectors you have to sit still for decent interval of time - some of that time is spent micromanaging your refinery - but there's plenty of time to switch to cockpit view - rotate pitch or yaw to line up with next asteroid short distance over and fire off a prospector.
usually, I can fire off 2 prospectors (sequentially - not concurrently - honestly the only part I disagree re: mining guide is I find more than 1 prospector useless - they self destruct in weird times and the cargo hatch goes crazy with all sorts of false messages re: cargo scoop not open, closes it for you, opens it, I would personally avoid more than 1 prospector, but 1 prospector is awesome.
anyway - while your collector drones in that short interval of time they are making 2-3 trips to grab all the chunks (and again, that's 2-3 collector trips despite me cramming in and using 6 collector drones at a time), fire 1 prospector at closest asteroid you plan mining next. It takes ~6-7 sec for the prospector to get there and give you reading - just target it using normal T command, no need to switch to left pane. If the reading is great stuff - palladium, osmium, then you know soon as your last collector is finished, you can head to that asteroid.
but like I said, I have time to fire off usually at least 2 prospectors during my wait for collector drones to finish - so if that 1st prospector scanned asteroid is crap - none of the top 3-4 metals, then just fire off another prospector at another asteroid next to it. You will usually have the reading on that back just as you are finishing last management of refinery and last collector comes in.
So that's about it - that 1 prospector lets me have very useful time saving intel whether to even bother with the next asteroid or not - and the limpets are super cheap.
I pack the ~20% with a little more cushion of collectors recommended in mining guide, so for my 160 ton python cargo hold, I pack ~40 limpets to be used as collectors, and then cram the rest of the entire hold with max limpets - all 160 tons filled with 160 limpets, because I will fire off a ton of prospectors.
I think mining can be just fine if you want to use the blast rock, target chunk ,see if it is kinda decent, then mine or move on, so no prospector needed.
But technically, since we are just sitting there for short interval between collectors returning with chunks, using a prospector at that point does save you a good deal of time ignoring non-top 3-4 metal asteroids and only spend time mining / collecting on the good ones.
