Hi Alfred,
Here's my test, done just now with Numbers '09 v2.0.3
Columns B, C and D were formatted as indicated by the label at the top of the column. (Column A is a Header column, formatted as Text by default, and was not used. After formatting, a number, 1, was entered independently in each column. (Not filled right, as that would have carried the formatting of the first cell into the others.)

All three cells were then selected as shown above, and the fill control dragged dow to fill rows 3 - 10.

As can be seen, only the Text-formatted column incremented.
"To say that I find this having to select 2 cells to prevent the increase somewhat inelegant would be to add positive spin,"
I don't see it as particularly inelegant.
There are three default behaviours:
- increment by one unit (for single letter text strings, numbers formatted as text, text strings ending in a number, and dates)
- increment by zero for numbers and durations
- do not increment text strings not described in the first set above..
For any value included in the first two sets, you can chose a different increment (including a zero increment for the items described in the first set) using 2 examples.
For any value type included in any of the sets you can set a repeating pattern using the minimum number of values to establish a non-incrementing pattern. (eg. a,b,c or z,y,x will increment a, b, c, d, e... or decrement z, y, x, w, v,,,, but c, a, b will repeat, as will a. b. c. a. b. c )
"Why, I wonder, isn't it possible to just select a format option that prevents the increase from taking place?"
Most likely a design choice.
But there is such a 'format option.' Format the value as a formula:

The text column has had its format returned to Automatic.
Formula in B2 and D2: =7
Formula in C2 (to get text result): ="7"
All three cells selected and filled down as before.
Regards,
Barry