I mention a couple of sort-of hybrid paths in my own leveling. I did this to increase my survivability without hampering my ability to deal damage (too much anyway). As I mentioned in the FAQ, I went down a path in Ret to grab SoC, and then went down holy until lvl 61. This pretty much just got me to Holy Shock then I had to stop.
The reason I did this was for survivability and mana efficiency. Before the nerf to Illumination, I would have all of the best Ret gear I could find, and therefore hit pretty damn hard. I usually wouldnt even have to heal in a fight with another class, I would just win. If I DID had to heal, I could force a crit holy light on myself, effectively bringing myself to full, and not have wasted any mana doing it. This also reduced downtime while leveling, as I didn't have to stop and drink that often. As soon as I needed a heal after grinding mobs, it was a free health bar every 2 minutes or so. This has been reduced somewhat by the Illumination nerf, but it still kinda works.
The drawback to doing this was that I couldn't put out as much damage as I would have if I was retribution. Aside from some crit bonus and slight damage bonus though, the 2 biggest talents to greatly improve your dps are Seal of Command, and Crusader Strike. I already had one, and wasn't even high enough to get the either, so it didn't really matter much. I never geared for int or spell damage or anything like that for the holy side, just grabbed as much strength as I could and the stamina that came with it.
My other hybrid I did at 61. I knew once I got into outlands I would want to tank. However, I didn't want to give up my current single-target strategy from the ret side. So I kept my same gearing philopsophy, but I changed my talents to 10/31/11, grabbing Spiritual Focus, Holy Shield, and Seal of Command. This allowed me to tank instances, retain my ability to heal myself through damage, and stlil have SoC for dealing damage. With Reckoning this allowed me to grind a little faster, not as fast as if I was full ret, but still well enough, and the tanking stats let me tank instances and solo group quests. I had to have a seperate set of gear for tanking though.
Anyway... this was just more of an elaboration on my talent choices as I leveled, I particularly enjoyed doing this, and it worked out for me. It's not for everyone, and if I had to do it again I probably just would have went Ret while leveling through outlands since I didn't tank as much as I thought I was going to.