single index on more than two coulumns a bad thing?

Hi,

I remember reading a post ages ago, maybe from Vadim, about the fact that
people creating indices on more than two columns will be the first to be
put againts the wall when the revolution comes... sort of...

Is it always bad to create index xx on yy (field1, field2, field3);

I guess the problem is that the index might often grow bigger than the
table, or at least big enough not to speed up the queries?

/Palle