Functional Language Summary

I've been hearing a lot about functional programming lately (and the circle of life continues); I found Functional Programming For The Rest of Us to be a nice summary. Here's what I got from it:

Atoms of FP:

• All "variables" are immutable, often called "symbols"
• Program state is kept in functions (specifically, arguments passed on the stack to functions), not variables
• Functions are first class citizens, i.e. functions can be passed as arguments
• "currying" is a convenience syntax for adapting a function to an alternate function signature
• "closures" are functions that are allowed mutable state and access to state outside their lexical scope to bridge functional and non-functional languages

Implications:

• Functions cannot cause side effects ("variables" are immutable)
• FP is great for unit testing (only have to test outputs against inputs -- don't have to test side effects)
• FP is great for debugging (no need to worry about external state affecting function results -- results are only based on the input)
• No need for multi-threaded locks, as state is immutable
• This makes functional programs automatically parallelizeable
• Can hot swap new function definitions w/o effecting existing instances
• Don't need to evaluate a function 'til the results are needed

FP sounds great! Why do we mess around w/ anything else?!?