You are right. Most coins and jewelry are in the top 6 inches. But that is only because there is more people now (higher popualtion) and more coins lost and more coins minted than ever before.
The kind of plcaes I hunt...depth is a lot. Let me first say that I hunt for coins as do the 3 other guys that I hunt with. Well, one them does a lot of beach hunting and water beeping too, but primarily we hunt for coins.
Garret's treasure imaging is great, but far from perfect. I'll tell you this, it has helped me in a lot of areas, but take it to a place where there is all kinds of B-sized targets made of old crappy metal and you will find that its no picnic. We have a fairgrounds in my town that has an old demo derby track, well actually its still in use, but the grade the area a few times a year, and move a little of the dirt around. Spend an hour in there and you will get a headache, because its harder than a rock to dig in most of the time and over half the metal in their gives you a belltone. I have found items as big as pop cans that were at 3 or 4 inches that told me it was a B-sized target. Have of the items you dig in there are bigger than a silver dollar. Some times it overloads (gives that constant buzzing sound) so I know not to dig them. However, I am telling you that this place is nightmare.
However, we hunt primarily old fairgrounds, schools, houses, and of course any downtown constuction going on. Anyway, most of the older coins are deep in these places. Garrett's treasure imaging is a sweet feature and I can't imagine some of the trash that I haven't dug over the past few years. Probably a ton of trash that I have not dug. However, the other guys I hunt with don't dig anymore trash than I do and they don't have treasure imaging. They mark trace the target with there pinpoint and it seems to work out just fine. The difference is that they get more consistent readings reported to their detectors, regardless of depth. I couldn't count the pull tabs I have dug at these old fairgrounds. I have only dug a handful at shallow depths (3" or less), but I have dug close to 200 this year already at depths ranging from 5-7". They were reported to me as a B-Sized target (and yes I was directly over top of it) and rang high. From a copper penny to a quarter. Some even higher. Rick (DFX user and Minelab now) and Jim (Explorer user) don't ever dig pulltabs.
They get a nickel reading at 6 inches and half the time its a nickel, which is good considering nickels are toguher to find than pennies, dimes, quarters, etc. The same thing at inches or less. There machines report more accurately than mine or Jon's (GTI 2500 user). Of course from 4 inches and less there isn't a real big difference in these machines accuracy, but once you start getting into deeper signals those guys are finding coins that don't even register on either of our machinces. If they do...they give some crazy trashy signal.
also, we hunted an old fairgrounds a few weeks ago and I found a silver war nickel, a liberty nickel, and two buffalos with the 1500. All of them were about 5-7 inches deep. By far that is the best day I have ever had in my life finding old nickels in one day. However, the only reason I dug them was because they came up between an Indianhead (1-2 notches lower than a zinc penny) and a dime. They popped off all crazy. I knew they were deep so I dug them. It was crazy, but still lucky. They were about the last coins that I was expecting to dig there.
You are right that a machine should not be purchased solely on depth, but I'll tell you what. The next macine I buy will be a Minelab Explorer II strictly for depth. These guys are finding coins literally twice as deep as I am. I was a huge critic and thought everyone was full of crap when they said they dug such and such a coin at 10 or 12 inches. Until...I have seen it done over the past year with the minelab explorer. I'll give you a story. We have this other old fairgrounds near us. This year we have been there 6 times. Jim ahd found over 30 silver coins, over 200 wheats and other goodies, all at least 6 inches deep. I found the nickels, maybe 20 wheats, and a Silver quarter all in the 5-7" range. Rick found nothing with his DFX cause they were all deep. He bought an Explorer II. His first time out he found 2 silvers, 22 wheats, and a buffalo. And he went there 6 times this year and found nothing deep. Depth matters to us in places that have been hunted hard or where the coins are deep. The thing is...we have only found 2 barbers out of all of these silvers. They are mostly Rosies and Mercs. My question is why the hell are they so deep at this place? And where are all the Barbers and Seated Coins? This place dates back to 1877. There is no way that someone else found them and left all the other silver coins. So you know they are even deeper and unless ground is moved around they probably won't ever be found.
My point is that every detector should be purchased according to what you are hunting. Garrett is easy to use, not too expensive when compared to some others, and it does have the treasure imaging. However, when I see what these guys find on a consistent basis with their explorers compared to Jon and I with our Garrett's...it would make you cry. That is all I am saying.