War or Sk, Enc or Bard, Cleric, Shaman, or Druid...are a fantastic core to any team. I'd go 4th dps if you have a bard. Note: the above formula is subject to how you play (Subbed-FTP)...as an example...a FTP Cleric or Shaman will be leaps and bounds better than a crappy tier 2 healer merc...
Free-to-play (FTP) has a whole different set of considerations. For example, you'll probably want to save up for some Kronos so you can max your tank/crowd control/healer asap. You can't force your way through with mercs in ftp...even a healer one. They'll be out of mana (wizard/cleric) in like 5-6 mobs flat in 105-120 content. If your tank is max gear/aa...a tier two healer merc can probably help...but you don't have reactive healing mode...so slogging it through nameds is going to suuuuck. Also, you can't do raid clicky based stuff without and actual healer on your team and you can't bring mercs to raids.
Best advice I got from the community is to decide now who are going to be your main char/chars and build your strategy around it. Rogue mercs are ok...FTP Zerker, Rogue, and/or Mage will still outshine a rogue or wizard merc any day of the week (even without aa's)
Best approach I've found. Max gear/aa's on your tank, dps, enc, cleric. Once all max aa's best gear...prestige gear only tank/dps and sub those accts, once enc/cleric are max aa...only gear them in non-prestige. A non-prestige max aa cleric, enchanter, shaman will rock out like nodoby business since they should never be hit. Tank/Melee/DPS classes are often gear dependent so that's where you want to focus financially so you can use/wield the best gear.
Hope that helps!