I do wanna pt this out there guys. Those that said a PriestRogue was a bad combo. Well in truth it probably is, but if you look at the bigger picture it ain't so bad. In fact I am now a 28 Priest/24 Rogue and I do finely with it. I do die sometimes, but not frequently. I have a really High Melee DPS and hardly get hit at all. Throw down Blind Stab and you got yourself a shield literally lolz. That ain't enough, how about the hefty bonus you get from both Agility and Nimble Hands which can be used because they are under the Rogue Skills not Class-Specific. I have high DPS with using 2H Staves. You can use a Mace and Shield, but you won't have as high as DPS intended to use.
On top of that you still have Bone Chill and Amplified Attack and Grace of Life.
I fully support your will to play "out of the ordinary" combos, and even taking unorthodox measures in playing them. But I'll be honest with you: you have very little experience to say how well your plan is going out for you. Not until you hit *at least* level 53-55 at bare minimum, where you'll have to quest in an area called Savage Lands. The linear scaling of the enemies' stats stops there, and everything gets increased by about 2x-3x higher. If you can get through Savage Lands with minimal trouble (I'm talking about solo'ing through it while on-level and quest geared) while maintaining your current path - then please tell me and I'll applaud you.
I don't have any intention of coming across as rude or arrogant or anything, but when you call out more experienced players and say "if you look at the bigger picture" and try to tell us how well you're doing but you haven't experienced much in the game at all... it irks me a little.
Now, with all that said... I do 100% agree with you when you say that the p/r combo isn't all that bad. Why? Because it is a really good combo, just not used as often as P/k (considered best priest healing combo in game) or as p/s (best PvP priest combo) or p/m (commonly used because both priest and mages are caster-classes) because the idea of having a "melee dps" class as a secondary to priest seems ridiculous. But looking at the elite skills you get from going as P/r, you get nice bonuses to healing in the form of:
-Quick Spellcasting (an elite skill that makes nimble hands increase spell casting rate) which allows you to pull off your heals quicker than any other priest
-Shadow Fury (an elite skill that gives a party-wide buff to magical critical hit rate) which allows you to crit on your heals more often, as well as boost other mages, druid and priests with getting crits on any spell
-Shadow Fairy: Now the shadow fairy doesn't help with healing. But it's aura increases the matk of everyone in your party by 10%. This is, imo, an awesome buff that every mage I've run with loves. The aura also gives the owner (in this case, you) an increase to all dark damage you do. This means that your magical-damaging dark spells that you get earlier (Cursed Fangs as well as Lure of the Snake Woman and Infectious Wound) all deal even more damage.
Another thing I want to point out that you're kind-of right on, is that the general skills of your secondary class do have the potential of making a specific combo really great. But that usually will only apply to classes that have similar rolls. The best example I can think of is having Warden as a secondary works GREAT with any physical dps'ing primary class, such as rogue, scout and warrior. Why? Because the Warden gives a physical dps% skill that is spammable and uses an energy source (mana) that differs from the primary's source of energy (rogue's using energy, scout's focus and warrior's rage), as well as a 20second cooldown spell that increases your Patk by up to 17.5% if i recall correctly.
Just remember though, that despite what I just said in the above paragraph - the main thing that makes a combo good is the elite skills that you're provided. The presence of useful general skills from your secondary can make a good combo even better, or a so-so combo pretty viable.
Sorry for rambling on, but I thought I could clear a couple of things I think you may have misunderstood about earlier.

If you aren't sure about something, or want to ask any questions or whatever, feel free to PM me.