I haven't found any content that absolutely requires mercs, given you have the minimum number of real players and appropriate classes. That being said, it depends on the real player's classes. Definitely by SoD, if not earlier, if you don't have both a real tank and real healer, you will need a merc for those missing roles. To help explain, I'll list my usual transitions.
Level 1-50+: Grab a J1 tank merc. If boxing, add in an App5 melee merc or anther J1 tank, depending on classes. If I played much casters (been thinking of 2 mages), I'd probably go with 1 tank and 1 App5 caster.
Somewhere between 55 and 65, I switch to one of the mercs to a J1 cleric. It depends on the real toon's classes. For a tank, I go with a clr. For melee dps, I go with either a rog, tank or clr, depending on whether I will be primarily boxing or not, and if boxing, what the group needs. Molo, it's generally a clr. For healers, I keep the tank. For casters, it depends mostly on what the groups I'm in needs (I don't play many casters though, and mostly to box when I do).
Generally, around 70-75, I start working on the Oceangreen progression to get Void B and Tier 2 mercs, as that theme is pretty doable by then if boxing with mercs, and upgrade when finished, if still in the 70s. If in the 80s by the time, I finish, I look for getting in on Warrens progression or doing the harder arcs myself, as well as doing any open raids (much faster progression doing successful raids). The goal is to get T5 and Void E or higher by 85. I don't have UF or HoT, so can't give really good advise on mercs in those expansions or post 85.
Getting a merc at 1st level is good for a couple of reasons: 1) It's free; 2) They level up with you for a reasonable amount even if you aren't using them; 3) tank mercs are severely twinked (other than dps) at lower levels [In comparison to real twinked War, the real one will be slightly better if it has good worn regen and descent or better dps weapons, and alot better aggro-wise with good procs (1 blade of carnage will out do a merc till 50+ for both dps and aggro)].
Clr mercs are not effective until the 40s when they get Complete Heal. Until then they burn up mana fast, especially trying to keep buffs up, and have low heals. Even to the 20s, heal potions are way better than a clr, as they will burn up all of their mana just keeping buffs up. I've heard caster mercs have a similar problem keeping mana up at low levels without crack.
At higher levels clr mercs are effective. They're not as efficient nor as responsive as an attentive real cleric, but still adequate and attentive. My real clerics, eventhough under geared comparatively (I keep mana regen and heal foci good, but have less ac, hp, and mana than merc ones), I have better aggro control, waste much less mana, and can respond faster to situational changes, as well as adding in some dps, especially vs undead, but even reverse ds on most mobs, unless healing times require chain heals. A very good Dru or Shm will out do a clr merc, assuming they have descent mana regen and foci to compensate for the Clr's more efficient heals. I really wish the clr mercs had toggles for reverse ds and stuns. Mercs not RDSing nor Stunning is a major draw back compared to a real cleric, as well as not casting lower level buffs.
There are some tactics you can use to get better control and response, as well as counteract low confidence (keep them from running away) with mercs. These tactics should be listed somewhere, and as I'm hungry, I'll not repost them atm. I may come back and edit them in later, after I get some breakfast! :)
Yther Ore.