Since Nginx started to add more and more features to their loadbalance it makes sense for HAProxy to get a scripting engine.
EDIT: I guess part of my concern with Lua was the example on the linked article. It seems to be able to interface with most of the system and not just as a content source, which makes more sense.
It's one of those "take apart your airplane and put it back together again" projects that one generally shouldn't do, though sometimes there is no way around it. If you need to rearrange the whole internal architecture of your engine, you should it do it quickly and not mix it with new work.