Let me try illustrate Radiant-style face drag.
To preserve precision in transformations, a face in Hammer is not represented by a normal vector + distance from origin, but by one triangle it is spanned along. If, for example, you move a brush around, a dependence on only the normal vector would quickly introduce numerical errors. The better method with more data enables us to simply offset the triangle of a face accurately.
In Radiant-style face drag, you start dragging (with brushes selected) at some point p0. And you move your mouse to p1. The triangles of every face that is facing
towards p0 are offset, all others are not influenced.
If you drag x to the right with this selection of brushes ([] is a box, p# is your mouse):
[ ] p0 [ ]
You get this, essentially "moving" the hole (should work with brushes above and below too):
[ ] p1 [ ]
In hammer, you'd need to select all face vertices for that operation, and that is very time-consuming, even if it only takes you a click and a key press.
Also, if you have such a thing:
____
/____\
You could preserve the slope angle at the sides but still adjust the height of this shape. (Yes, it looked better in the text editor without this big vertical spacing.)
If you have modeled some stairsteps, of stairs where every step ends at floor level, you could copypaste those steps to another position and then face drag either side (up or down), extending the stairsteps' height.
Hammer's way of "hiding" the brush-defined-by-planes paradigm is certainly easier for beginners, but I feel mapping is much more comfortable if that paradigm isn't hidden, but exploited. In brush math, an arbitrary clipping plane is very easy to realize too (hey, hammer doesn't have that either).
Regarding brush merge: Yes, it is useful for spheres, but it also speeds up creation of such simple shapes:
[img]http://www.intercomm.com/shrinker/projects/Microbrush/clipped_t.jpg[/img]
Here, I created one side of the brush, conducted three clips, then mirrored this part twice and merged the four resulting brushes.

(One aiming with the mouse and a key press for centering the grid there, then two key presses for mirroring the selection twice, then one to merge it all.)
I occasionally run across a situation in mapping where I think "These two could be merged!", and in practice I really do that, when not using Hammer.
Shrinker @
Beyond-Veils,
InterComm