Using DD-X at 1:9 allows for a greater margin of error, due to the longer development time. It's a "gentler" way to develop your film, I would say.
Another upside is that you can use less DD-X per film, which means you're saving a bit of money. DD-X tends to be one of the more expensive developers on the market. This may or may not be a consideration.
Downsides? Well, naturally it takes longer to develop your film. Another potential issue is grain ... longer development times combined with weaker solution can result in larger visible grain on the negatives. This wouldn't be an issue at lower ISO's, but might be a consideration at 1600 or above.
I'm by no means an expert in film / developer chemistry, but from the knowledge I've gleaned over the past few years, this is what I've learned.
Basically, if you wanted to minimize the grain of your high ISO film, I wouldn't develop in DD-X at 1:9.
In general, I liked the consistency of DD-X. It's a very good quality "general purpose" developer, and easy to use as well. FWIW, I've moved to XTOL in the past year, only because I like the look of XTOL better. It has more of a unique signature than DD-X, and it's much cheaper.
