The original post was to address the comments that Koldoon wrote in that the area of the map is not generally adjustable and that some of these programs run very slow both of which is generally not so of ViewingDale though it depends on the graphics card used. I have a 600Mhz machine with an NVidia GeForce2 which is a card too old to buy in the shops and can be found on ebay for a just few notes. This machine runs it quite well though not as fast as my newer machine of course.
All of the points that you bring up are valid and you are correct in that you cannot draw freehand, or do patterned flood fill's etc in ViewingDale and do need to import your custom images from another source but it does give many important benefits that Photoshop cannot do.
Fundamentally, photoshop and similar art packages are bitmap editors and so you have to decide on what scale and area you want your map when you start to draw it. You can make it larger, though there is a moderately strict limit as to how much larger you can make your map due to memory constraints - especially if you want to print it. ViewingDale is able to store a vast expanse of thousands of miles and keep the detail for it at any scale. To have a bitmap of a 5 mile square map at screen res (72dpi) and at miniature scale (1:72) would take about 350 Gigabytes. At 300dpi and with 512Mb of memory you can fit a single layer map of just 77yds square.
Once you have decided on your blank map area in Photoshop you need to draw everything on it. If your smart you could draw bits of it seperately and save them off to use them as brushes to stamp down duplicates of that brush such as cart or wagon for example. If you want to use that brush in the future then you had better draw it high res and scale it down to your current map scale otherwise you will never be able to use it for a map that is of smaller scale than the one you are drawing.
Next, you had better place all of those brush templates on a new raster layer otherwise you won't be able to change your mind later on and move them again. Every extra raster layer increases your memory requirements for the map further restricting your size that you can work with and it also means that your PC has to be a monster if every brush has its own individual layer.
Then once you have meticulously drawn your map doing all of that you can only reuse that map as a source brush for a larger scale maps and never for smaller scale ones and if there is ever a requirement to print your map at a resolution higher than you originally anticipated then your stuffed.
If your map is one that is part of a larger landscape then the next map you draw is going to have to line up with the first exactly. You can take one edge of the first map and copy it to the next but you will have lost all of your layers to do it. As an example of this look at this image.
http://img518.imageshack.us/img518/1576/todsfrontier0fr.png
This is the edges of two Frontier maps as from from wizards Map a Week by (I think...) Todd Gamble.
http://www.wizards.com/default.asp?x=dnd/mwa/archive2003
As a seriously excellent map maker and Photoshop Whiz, these guys are still having difficulty getting the maps to line up.
ViewingDale doesn't have fixed map edges at all so its a non issue. Since its not a bitmap editor and, as you say, more of a collaging program, then it doesn't have to use layers. All of the images can be reused and have a high probablility of being already drawn by somebody else. Therefore, if those images were to be shared then everybody would not have to keep drawing the same stuff over and over again. The argument for using Photoshop over any other kind of mapping program is similar to asking why anyone should use Photoshop when you can use a pen and piece of paper. The whole point of a mapping program, surely, is not to have to hand draw the map in the first place and if there is a way that you can get your custom player maps without having to do any (or at least most of the) freehand drawing then its got to be worth looking at.
Finally, ViewingDale is a zoom browser and information linker. You cant jump to the interior of a cave entrace using Photoshop, you cant add photos and documents to the map in Photoshop. In ViewingDale, every container can have the capacity table linked to it, every monster have their stats attached etc. And as for cost, Photoshop might be great value for money but its also a whole lot of money too. Amazon.com is selling it for $550 - though programs like The Gimp are free if not quite as fully featured - if your still inclined to draw your maps freehand.