tsearch: immutable functions?

I was wondering if there is any reason not to define the functions from
tsearch as immutable. What I am trying to achieve is that I don't have
to create a separate field to index as is explained in the manual, but
just create a functional gist index.

Instead of:
alter table titles add titleidx txtidx;
update titles set titleidx=txt2txtidx(title);
create index t_idx on titles using gist(titleidx);

just:
create index t_idx on titles using gist(txt2txtidx(title));

But creating a functional index requires an immutable function.

Jochem