Well, spirit visage doesn't provide Spell Vamp, but I know what you mean :P
Anyway, he is underplayed because:
- Lacks mobility.
- His spells have short ranges (makes him better toplaner than mid IMO).
- His spells are all DoTs (except W). Players underestimate dots.
- His W is hard to land against competent opponent.
- He relies heavily on sustain from his ultimate. If it can be outdamaged (burst damage usually) or reduced (Ignite, Morellomonicon, Executioner's Calling (Haha, I'm so funny xD) or champion abilities with Grevious Wounds (Fizz W, Kata R, Tristana E etc.)) then he dies fast.
Also, while I do like using Exhaust and Ignite as a combo, you might need that extra mobility on Swain to escape or catch up (depending on enemy team it might be better to pick Ghost instead of Flash though). And both E and Q are good harassing tools, with E being the better choice for maxing first (pretty much regardless of situation) as it will deal more damage than Q (and will also make Q deal more damage, making it a big difference).
From a simple E->Q combo at level 9 you will have:
426 + 200% AP damage if you maxed E first
and
411 + 189% AP damage if you maxed Q first.
It's because E damage amplification works on E itself, making it a better choice unless you REALLY need that slow to be bigger. Also, E damage cannot be interrupted unlike Q and it will increase damage from his W and R too, should you use them.
