I've been involved in a blog discussion about the Angel of the LORD. 94 comments and still going strong.
I'd like to clarify a couple of things about the Angel of the LORD:
The Angel of the LORD is not by any means the only title under which Christ appears in the OT. He also appears simply as 'the LORD' or as 'the Word of the LORD' or 'the Commander of the LORD's host'. Christ has many names.
I don't believe that actual appearances are the only meaningful ways of talking about Christ in the OT (there's also the promises and types).
When you hear "Angel" don't automatically think 'creature'. Literally the name means 'Sent One' and it has come to mean 'Messenger' (good titles for Christ!)
There are lots of other angels in Scripture. But just as there are many 'sent ones' from God - THE ONE Sent from God is a divine title.
Now. Why bother talking about Him?
Let me just give one reason for now (I speak at great length here on the subject):
When you realize that both OT authors and OT saints appreciated the divine identity of the Angel it forces you to rethink a kind of default unitarianism innate to much of our OT understanding. Once you know that the Angel is both of the LORD and is the LORD, He is from God and is God, distinct and divine, you see that OT saints did have a conceptual framework which allowed for conscious faith in the distinct Person of the Sent Mediator.
If that all sounds like jargon, later on I'll post something much simpler.