I ultimately came to a similar conclusion. I've started construction on a permanent layout, which I too will be powering with digitrax equipment. My decision is based on a couple reasons. One: Its cheaper than using a series of Ring power supplies, although it does not rival the power method put forward by Alan. Two, I'm alone among my friends in the Railpro venture, DCC runs strong with them. By having a DCC power supply, they are able to run something of their own. With the newish Wifi interface that Digitrax has, I don't need to invest in DCC throttles. Third, realistically I will never convert my entire fleet to RP. I own somewhere over 100 locos, many already have DCC. It would not be cost effective to convert everything. Also, as Jacob mentioned, there are cases where conversion is basically irrelevant. For example, I'm planning on a solo RDC run for my layout. It would be just as effective to pony up the minimal cost for a sound equipped Rapido than go through the hassle of buying a DC, converting, then never consisting it to anything. Especially when there aren't really any correct RP sounds.
I really think this arrangement covers the best of both worlds.