cases environment requires the
For historical reasons, LaTeX defines a
\cases macro: on page 232 of Lamport's manual one reads
Most Plain TeX commands can be used in LaTeX, but only with care.
\cases is indeed a Plain TeX command which has quite a different syntax than the
cases environment. It would be possible to use it without loading
amsmath, but the features of this package are much better than the Plain TeX commands: more flexible and with a uniform syntax.
By the way, here's how one could input the equation with this method:
x + 1 & blabla \cr
x & otherwise \cr
It's evident that the syntax is foreign to LaTeX: the second column is typeset in text mode and not math mode and the lines must be terminated by
\cr. Most important, the alignment must go between braces: this is the main cause of the error.
Don't use it and stick to