I'm looking to changing up my army a bit and will try to incorporate these deadly skaven contraptions. Let me show you how I think.
First of all. The negative side of the thrower are their cost and horrible misfire chart. Against some armies with much small arms fire, these are hard to keep alive and sometimes they will just blow up on their own, if you're unlucky taking out swathes of your own units. This means you risk giving away free VPs. As such you need to mitigate their weaknesses. The main ways of doing this are:
Neutralize enemy shooting
Destroy/tie up shooting units. Mainly small shooting units like fast cav and shades as they are most likely to shoot them as they can't hurt anything else. If your opponent soaks an large shooting unit or artillery into the thrower it will die, but have taken away considerable shooting form your other forces. You might want to take some flank field protectors like a bigger giant rat unit or a doomwheel to chase away all the annoying support units. You might also want to consider the storm banner.
Minimize your own shooting
Yeah, you heard right. You actually want to shoot a very few times with your throwers as they will blow up, with potential catastrophic results. I believe these shoul be used as a terror weapon. It takes som gusto to move up with an expensive unit within 12" of your team. As such the opponent tend to be cautious when advancing on you. Set up the shots as good as possible to take that utterly devastating hit rather than try to long rang shoot just because you can. I'll try to show an example on how to set up a shot.
Here we have a skaven formation straight opposite an 25-man unit of whatever. This picture is set up as the opponent would have moved up. About 10-12" away from your stormvermin in this case, and about 15" from your thrower. This would enable him to charge and to stay outside the effective firing range.
This you may counter with an ultimatum set-up. There are plenty of these situations and meaning an situation when the opponent may choose to do something he don't want or nothing at all - both not optimal. Lets' look at the picture here.
Some things of note. As your throwers misfire are deadly, you might want to shield your units from them with an intervening rat-dart or terrain feature to prevent the team running into your lines and immolating in an 5" s5 blast...
The throwers may of course be used in other ways by locking units with slaves etc. Though I am very wary of firing into combat if I may kill many slaves, as the important thing is to hold. However you get my point of use for the throwers.
Deathstars beware, an skaven army with 2-3 warpfire throwers plus doomrocket and warp lightning cannons can obliterate almost anything. Something ought to hit ;)
Feel free to comment if you've tried the warpfire to good or bad effect.
I will at least try to make use of them. We'll see if they make it into my tournament army for the autumn.