Lowbie healers DO need guard, because they are effectively in DPS gear. Healing abilities have a threat multiplier attached so that they draw aggro when using alacrity gear, to make things more difficult. In lowbie gear, where alacrity is a lot more rare than say Surge, Power, and Crit, they are putting out a lot of threat. This goes down as healers start to stack Alacrity, and their other stats go down in comparison to their DPS counterparts, so it works out that they put out less threat than most DPS.
In lowbies, guard the healer. In endgame, guard healers that are tardic enough to wear DPS gear. They need the threat reduction that Guard provides.
So many bads...
Your "facts" are made-up. Heals draw 50% of the threat that a similar attack would do, unless the attack is a melee one, in which case heals generate even less threat. Alacrity, if anything, causes a healer to generate slightly more threat because it will cause their HPS to rise. It is not a threat multiplier either way. Mobs have to simply be un-engaged by anyone to agro a healer who is doing his job.
Guard the melee DPS, or lacking that, the DPS most likely to pull agro. If the healer (or other group members) whine about it, educate them. If they refuse to be educated, that's fine (some people have short times to play and want to get things done), but don't just capitulate because they whine. Make sure you educate DPS on proper kill order while you're at it!
And this advice doesn't go for top-end elite raiders. They know what to do with Guard. I'm giving advice to tanks for leveling FPs (and HM FPs, for that matter).