Completely agree with what Kumudil has posted. With one addendum. Merc tanks are stupider than a rock with a lobotomy.
I currently 3 box, and have dropped my druid (was shm/mage/dru as my 3 box with monk sub in for mage when friend that I play with wasn't available) and am building an SK to replace.
SK are AMAXING. I can't believe the stuff they can do. It's so much fun to play one. No disrespect to the Paladin folks, but I have a 82 Paladin with full SOD T4 (also on mage/monk account) and it just wasn't as much fun.
Granted, another reason I am building the SK is to 3 box, and keep reasonable DPS available (via mage). The monk is a brilliant puller, but is a sad panda dps provider. The SK can reasonably pull, and this allows me to bring mage back in which I prefer over monk.
Sadly boxing Paladin I really just found myself, /autoattack /stun/stun/stun.... /stun. /weakheal... Paladins frankly need some serious love... (this is a topic for other posts, but I truly believe this now. Enough to sideline a 900 AA lvl 82 paladin to build an SK)
SK has the opportunity to provide 3 of the skills required in a group.
-tanking
-pulling (although this one is still a bit sketchy but the SK seems to have some nice tools to do it with)
- runner control via snaring.
Also in a RUH ROH! moment can flop him over, wait till the ** clears the fan and pop out your merc healer for /'rescue me' . Now of course this isn't as important in the modern game but still pretty damn nice.
Your necro can provide all of these except the tanking part (and that is debateable in a lot of older content) and the merc tanks are BRILLIANT against a single target, but god help you if you get an add.
With my typical set up, this is what getting an add looks like:
I VP add. Merc tank goes running over to the add, making sure that the mob he was fighting has a nice clear shot at his back with no mitigation now... I get the stupid merc back on the original mob (sometimes..) and then the add can pound away on him from behind. This is with an assigned Main assist. Merc tank just doesn't care, he must have agro on all at all times. In fairness that is advantageous in a lot of situations, but the one I described above happens often enough that I am building a Sk tank.
Don't get me wrong. I love merc tanks, but they have some serious drawbacks.
Ride free brother.