Compare the price of a long arm to the cost of paying to have quilts done by someone else. If you can still rationalize the expense, and can afford it, then go for it.
I plan on getting a mid-arm (approx. 11" harp space) to put on my frame so I can quilt my quilts more easily than I now do with my domestic sewing machine (approx. 7" harp space). I know I'll never quilt enough quilts to justify the expense, but I really enjoy this part of the process. So if it works out that my future quilts - including the cost of all my equipment - average out to be $500 or more per quilt - at least I've enjoyed myself. And if I get good enough at it and end up doing a few "contract" quilts and make a little money, great!