Paladins use to stack a spell damage weapon to maximize threat -- that isn't needed anymore. Using a "tanking sword" offers much more than a spell damage sword. They typically have hit/expertise/avoidance/str/stamina on them, which is always a boost over spell damage weapons that have haste, crit, and int in wrath. Not to mention, all spells scale off AP and as spell power equally, and since 1 str = 2 ap, you need over twice as much spell damage on the weapon to give the same damage boost.
The tanking weapons are also much higher base dps -- 143 dps for the red sword vs. 81 dps for the spell damage weapons, and remember, HotR scales directly off weapon dps, so the higher the weapon dps the higher the hammer hits.
i agree for the most part, however i have found that with a spellpower wep my threat scales far faster. the only spellpower item that i currently have is the titansteel defender (please forgive me im new to posting in forums im not sure how to link to wowhead yet). since ive gotten that ive been hitting between 2.5-3.5k tps on trash and broke 4.5k tps on patchwerk in 10 man nax yesterday. if your having problems with threat i would definately pick up a spelldmg wep over a "tank" specific wep.
Also comes into the factor that only on trash are you doing higher dps by using a spell damage weapon. On bosses you are doing lower DPS since melee does not gain the full threat modifier holy spells gain.
Short answer get a melee weapon preferably with tank stats but also get a healer weapon when you can and use it for trash (though depends on loot system).