1. Two reasons actually. First if something DOES manage to break past your lines and head straight for your Slaan they will have one hell of a fight on their hands as well as being able to function as a fall back hammer unit incase things go really wrong (template attack wipes out your skrox, fliers/infiltrators/diggers come at your from your flank, ect.). Secondly, and this is an odd one, incase of miscasting. Though it is rare to have a miscast effect a fully geared Slaan it is not impossible. I once had an Ogres players use his power dice to dispell my throne of vines from lore of life then upon my attempt to recast I rolled the double 6's. Sadly, the miscast roll result was a 7, Detonation.. all models in base contact with the mage take a strength 10 hit. I lost 9 temple guard to that miscast BUT since I started with 18 I was still fighting fit. if I had gone with only 10 I would have been a sitting duck. Also, bear in mind, there are armies that can force a miscast on you (Ogres, being the one I face regularly, have the Hellheart for instance.) so the fact our Slaan have such a huge base can work against us.
2. This is why I field the scar vet minimally with light armor and great weapon. His sole purpose in my list is to keep the Slaan in the fight in the case of a necessary charge. Allow me to explain, lets say you're taking Orcs and Goblins and a unit of Orc Boar Boyz with an Orc Warboss gets past your front line and charges your Temple Guard. You hold since the threat of over run is too great and he declares a challenge. If you have a TG champion then he could answer the challenge but he will be dead by the end of the first combat most likely. This buys you one turn of 3 strength 6 attacks against his boars to try and take combat resolution (since that means -3 to armor saves it turns their 2+ into a pitiful 5+). Next turn, if he doesn't break, the Warboss can issue another challenge which you can answer with your scar vet. This tangle could take a while due to comparable stat lines and might come down to luck of the rolls.. but even then this buys you a whole extra turn or two. The point is that if your Slaan answers a CC challenge it is gonna die FAST... if it refuses the challenge it moves to the back rank and now cannot support your army with it's leadership.
Edit: The rule actually says it has to "move the model so it is in a rank that is not in base contact with enemy" so he can stay happily settled in his second rank bunker.. though the inability to share leadership still applies.
3. I chose to not use salamanders due to the fact I was fighting ogres in 2 out of my 3 fights. You can never go wrong with salamanders they are just rather unreliable specifically against ogres. They should perform wonderfully against WE due to small bases, small number of wounds, and being flammable on some of the models.
Actually, Maazie, nowhere in the challenges rules does it say the a characters must be on front rank to be involved in a challenge, you target the unit. If you have a character in the unit and you refuse the challenge they are taken out of the fight. TG champions will buy you one CC, maybe two if your opponent is having bad luck with rolls. Besides, if you really want a scar vet in your unit of saurus then I'd suggest an entirely different build. I usually roll my saurus warriors as an anvil I'm prepared to sacrifice if things go sour.. their sole purpose is to tarpit strong enemies and roll through weaker.
Also, side note, earlier I stated something incorrectly. Specifically under the Slaan rules it states that he CAN fire magic missles into whatever combat the TG are in as long as he is in the second row. Lore of Death just got a lot more interesting...
