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Hey neckbeards talking about your super logic "I must know the truth" crap.
Having played an SK to 60 in a past life I can answer a few of your questions and more:
1. The character will never be played solo.
Your choice of warrior or SK has no implications for this statement. Both classes do well in groups. SKs have better survival and utility (FD, IVU, Invis, Pet, Snare, Fear, Taps) as opposed to warriors which have....well...more hp/AC. SKs have it easy-mode to get aggro and keep it.
2. The vast majority of its playtime will be duo with a shaman.
As it has been highlighted, SK/Shaman are like a staple duo as their ability in large ways complement one another. The extra DPS from both of your pets is also a plus.
3. Once in a while the two of us might get into a full group.
You will be an excellent tank for this full group. Your shaman will also be welcomed for slows/heals/buffs.
4. I will almost certainly never raid with the character.
Good. If you were looking to MT raids then you should go warrior. SKs typically play a backup tank in raids (although there are exceptions).
5. I have a 50 necro to farm gear/plat for my warrior.
This makes me believe that you should definitely go SK. Many of the spells that SKs get you will be familiar with and know how to use effectively.
Lifetaps
It is true that before Kunark SKs have pretty shitty taps (the lvl 49 life leach is pretty good but it has a long reuse timer). However, in the 50's during Kunark, SKs get "Spirit Tap" at lvl 55 and "Drain Spirit" at lvl 57, which correspond to necromancer's lvl 29 and 39 lifetaps respectively. These are pretty nice taps at 150+ to over 200+ hp per cast. Combine these with shaman heals and you're ready to rock.
Deeps
SKs get some fairly nice dots (lvl 39-49 necromancer dots) towards lvl 60 that stack nicely with shaman dots. Although they are not overwhelming strong dots, combine them with your pet (which your max lvl pet is the lvl 44 necro pet), and your melee damage, and suddenly you're dishing out some respectable damage.