Q:

Why is the Alternative symbol of the ECMAScript RegExp grammar left recursive?

I cannot for the life of me figure out why Alternative is left recursive. It really throws a wrench into my parser.

Alternative :: 
    [empty] 
    Alternative Term 

Here is a note in the semantics portion of the spec that is not exactly clear. Maybe the reasoning would be revealed once I understand this?

NOTE Consecutive Terms try to
  simultaneously match consecutive
  portions of the input String. If the
  left Alternative,  the right Term, and
  the sequel of the regular expression
  all have choice points, all choices in
  the sequel are tried before  moving on
  to the next choice in the right Term,
  and all choices in the right Term are
  tried before moving on to the next 
  choice in the left Alternative.

What kind of parser can properly handle a left recursive grammar?

A:

Because for certain types of parser left-recursion is much better (e.g. for yacc - see section 6.2 here for an explanation).
If it's causing trouble for your particular parser, then by all means swap it over - it doesn't affect the definition of the language in any way.

