I wear hats from both camps as they say,i have 2 main machines that i use and both have had very high hours in learning everything about the machines and they have both served me well over the years,but although they are both very good all round machines,on some site and detecting scenarios then neither are the best machines for the job,so i do have a fair few additional detectors that excel for some of my sites.
I have one site that is roman trading villa site that had activity on the site for 400 years,so with the amount of massive activity and being inland you would have thought that a normal detector would have been a walk in the park !! wrong,this site is about 70 miles inland but the site is just basically like black beach sand and the only machines that can touch this site is mainly a Pulse machine,this basically cuts through the ground like a knife through hot butter,but no other VLF machine would touch this site,even multi freq Minelab machines struggle on this site.
Also on say a trashy saxon site,some of the older Tesoro machines with a small coil on are also the best detector/coil combinations for the job,deep pasture sites i find that i require a big coil combination etc etc.
In theory one car should do all your needs,but if you want too go off road shooting or detecting then a 4x4 is a good choice,for a big family run around a estate car would be the best choice,but basically they are all cars with engines and 4 wheels but some do a better job than other,so although o will totally agree with using just one machine,sometimes having multiple detector and coils can up your game plan in your favoure,must admit i am lucky that i do have a few machines for site specific use.