Q:

Why should you avoid "stringently columnar" input formats?

Doug McIlroy, the inventor of Unix pipes and one of the founders of the Unix tradition, had this to say at the time [McIlroy78]:
(ii) Expect the output of every program to become the input to another, as yet unknown, program. Don't clutter output with extraneous information. Avoid stringently columnar or binary input formats. Don't insist on interactive input.

Stringently columnar data sounds good to me, so I probably do not understand what he meant.
What does it mean and why is it bad?

A:

I assume he meant aligned columns, not columnar data in general. That's how I would understand the stringently anyway. For example:
Bad:
1    200 3
100  3   400

Good:
1 200 3
100 3 400

In other words, make files that are easy for computers to read, not for humans. Adding spaces to align things makes them pretty and easier for you and me to understand but can confuse programs that need to parse them. 
For example, if I were to use cut to get the second field of each of the above examples, it would fail on the first:
$ cut -d' ' -f 2 bad

$ cut -d' ' -f 2 good 
200
3

Because of the extra spaces, the 2nd field of the bad file is a space. However, it works as expected in the good file. 

