So the "major" GT scene is split this year on the issue of Forgeworld. Events like NOVA, 11th Co, BFS, AdeptiCon are going without it for their GT's ... events like BAO, WGC, FOB are going with it.
The most important thing to lead off with is this:
There is no correct answer to this question.
There are valid and invalid reasons for holding each position, certainly, and that's mostly what I'll ponder about ... but there are no correct or incorrect answers to the simple question: Should FW be allowed in your GT.
No matter what you do, the internet punditocracy is going to have its own thoughts about whether you should or shouldn't.
With regard to NOVA, it was a fairly large internal discussion with a lot of back and forth ... here's what pushed us to having FW be in the Trios and Narrative, but not the GT or Invitational:
1. Evidence came out over the year from UK testers that was fairly definitive as to the subject of playtesting - FW isn't playtested, or even involved in the balancing/playtesting process (such as / whatever it is).
1a. The game is increasingly balanced for all levels of play, with each new updated codex ... lending more credence to NOT including (in the more competitive formats of the GT and Invitational) non-tested/balanced rules.
2. None of us could remember ever really playing pick-up games at local game stores, social game nights, or local GW's where anyone - especially casual players - was even routinely aware of FW, much less interested in playing with FW rules. The push to use them was coming almost exclusively [at least on our radar] from more tournament-savvy people (though not exclusively from competitive minded people).
3. MOST gamers, even those who advocated FW heavily, weren't even aware of the existence of half or more of the units they wanted to legalize. As a result, many of the arguments about balance fixated on Sabres, Boarding Flyers (w/e their name is, I always forget), etc., and they consistently wound up stammering when faced with units like Quad Guns, or the (can't remember his name) Shrouding/Disordered + Nullzone/Divination 160 point libby, etc.
4. Arguments about "balancing" flyer-heavy armies were rendered somewhat pointless by the broad lack of consistent tournament-wining success by flyer-heavy armies.
There were a number of arguments for and against exclusive of those, but those seemed to consistently crop up more commonly.
On a personal level, I don't care all that much ... the only thing that gets to me is the lack of open, blatant admission of desire on the part of those who are really pushing hard for FW. There are plenty of players who want to use their cool FW models ... but they've been pushing to legalize FW for years, long before 6th edition.
Truth: Fifth edition was very vehicle-heavy
Truth: FW doesn't have all that much that's dramatically or meaningfully better than stock standard codices in terms of anti-vehicle
Truth: Fifth edition didn't have allies, and MOST of the really powerful FW stuff is IG
Truth: Now that MOST of the armies in the game can ally with IG in some way, the more powerful FW units are now more broadly available
Truth: Sixth edition is more infantry-heavy, and more flyer-heavy.
Truth: Forgeworld units - especially IG (see above) - are quite exceptional at killing flyers and infantry (Especially mass infantry, see: quad gun) ...
None of these factors are considered on a regular basis by truly casual or majority gamers. They are considered by power gamers ... by meta-reacting gamers ... by people who want to bring the most powerful army possible to the table.
I'm often one of these guys ... and there's NOTHING WRONG with admitting that you want to use FW b/c it gives you powerful answers to big infantry formations, and to flyers.
That component of the argument didn't really impact any decision making on our part ... BUT I see a ton of people out there trying to come up with all kinds of oblique ways to argue for FW, when at the heart of it for a LOT of them it's simply that FW gives them a lot of potent options, and they want to use them.
It's always a hot button issue to discuss, but what really is the motivator?
One of our local more prominent competitive gamers wants to go to WGC this year. He really wants to take a bunch of sabres, vultures, and quad guns. I could almost swear he's more excited to go to WGC as a direct result of FW legality. BUT his reason is not b/c he thinks FW is cool ... but b/c he believes most poeple wont' be taking FW, and doing so himself gives him a huge natural advantage over them.
Food for thought and discussion ... acknowledging (and don't go at me on this one, please) that there's NOTHING wrong with legalizing FW at a tournament, and it's totally just a tourney-by-tourney decision ... what are the REAL reasons it's being pushed by so many more players this edition who are loud and vocal on the internet (many of whom are known competitors)? Is it being pushed so much more b/c suddenly GW has done something or made some big announcement that it's meant to be included in most types of games ...? OR is it that the shift in the meta of 6th edition + allies inclusion now aligns the powerful units OF forgeworld with a much larger # of players ... all of whom naturally have a conscious or subconsious inclination to try and get whatever boost up they can within a brand new gaming environment?
There's nothing wrong with any answer ... but there is something wrong with judging people for it ... so try to keep discussion focused on trying to figure out WHY people want it so much all of a sudden ... and not on whether you think they're smart or good or wrong or right as a result.