It's not as effective. the main draw of /rdm is self-stoneskin + self-phalanx. PLD, having native enhancing skill, is one of the few jobs that benefit greatly from using phalanx as /rdm.
You can get by with Phalanx II merits, but the lack of stoneskin will be noticeable.
Actually, having done both I found pld + rdm to be faster and easier because I could aggro way more pucks. When their attacks line up, I could easily cure myself on pld, and when they were too spread out, rdm steps in. Between defender, convert, phalanx 2, chivalry, sentinel and hasted reprisal, I never had to stop, and 4 mobs wailing on you certainly speeds things up.