Why is left recursion bad?
Left recursive grammars are not necessarily a bad thing. These grammars are easily parsed using a stack to keep track of the already parsed phrases, as it is the case in LR parser.

Recall that a left recursive rule of a CF grammar $G = (V,\Sigma,R,S)$  is of the form:

$\alpha \rightarrow \alpha \beta$

with $\alpha$ an element of $V$ and $\beta$ an element of $V \cup \Sigma$. (See the complete formal definition for the tuple $(V,\Sigma,R,S)$ there).

Usually, $\beta$ is actually a sequence of terminals and non terminals, and there is an other rule for $\alpha$ where $\alpha$ does not appear in the right hand side. 

Whenever a new terminal is being received by the grammar parser (from the lexer), this terminal is pushed atop the stack: this operation is called a shift.

Each time the right hand side of a rule is matched by a group of  consecutive elements at the top of the stack, this group is replaced by a single element representing the phrase newly matched. This replacement is called a reduction.

With right recursive grammars, the stack may grow indefinitely until a reduction occurs, thus limiting rather dramatically the parsing possibilities. However, left recursive ones will let the compiler generate reductions earlier (in fact, as soon as possible). See the wikipedia entry for more information.