Well, for one reason, feet is clearer because not every game is based on a grid.Er... How so? How is one measurement any better than another when it comes to learning the game? I mean... I could easily argue that cubic feet is a terrible measurement. It doesn't have a clear meaning because of the infinite vagueness of dimensions and space in a tabletop RPG. How is even the DM supposed to know exactly how many cubic feet are in any given area? How is a DM supposed to know how these 20 cubic feet will be distributed? More important, how is the player using these rules supposed to know all of this and factor it into the game?
On the other hand, "Burst 3" is a pretty clear description. It doesn't have ambiguity or guesswork. It makes use of squares, which are a very natural way to count distance in a game based on a grid (as anyone who has ever played chess or checkers could tell you). It is not like the typical new D&D player hasn't ever played a boardgame or videogame before. Game terms are probably easier for new players to use than plain language when it comes to understanding game rules.
When it comes to lengths and distances, I prefer to use real-world units. Furthermore, if it's something I have to estimate or guess, I prefer imperial measurement.
However, my point was more about the word 'burst'. What is a burst? It has to be defined by the rules. It also gets confused with blast on a regular basis, but that's a different topic. Whereas, 'cube', 'circle', 'cone', 'sphere' these are all things that the players know and understand without having to learn rules.
Squares are great if you're looking at a battle grid. However, if you don't play with a grid, and the battles occur in your imagination, feet and yards are infinitely better than squares. Squares don't exist. To convert back to mind-space, you have to multiply by 5 and so on. I know that's not much in reality, but it's the same as the reverse argument. "How does a player relate feet back into squares? By dividing by 5". Is that hard? No. Is it annoying? Yes, for some players.
Perhaps the entries should say "30 feet (6 squares)"
Except that:GreyICE said:Burst 3 - 3 square radius, centered on the player
a) it's only centered on the player if it's close
b) when compared with blast, the numbers confuse people, because the two areas behave differently *
c) Burst has to be defined by the rules, while circle does not, because players know what a circle is
d) Square 'circles'
* blast 3 = a 3x3 square. burst 3 = a 7x7 square. Tell me that's not confusing when you are trying to quickly glance at your powers and know the size of the effect.