More and more of the lists I am writing these days include Bastions. All 3 of my armies, (Dark Eldar, Daemons, and Dark Angels) have uses for them. How many times have FMCs been caught on the ground and you lose one or two off the bat? Or units that rely on psychic powers or jink saves to survive but without first turn there is no way to get those up? This is why people bring Bastions. Its one major Drawback is its size, it is a big model, not very fun or easy to transport, (especially for those who use public transportation ) but it is becoming a staple for quite a few armies, perhaps almost as much as the Aegis Defense Line.
The biggest advantage to bringing a Bastion, is its curse, its size. At approximately 100pts (give or take 20 depending on upgrades) it is a place-able LOS blocking Terrain piece, which is invaluable with the current abundance of high strength ignores cover weapons(Wave Serpents). Anybody that doesn't want to get alpha struck, or rely on TOs to provide the LOS terrain, (some large events may not have very much more than 1 piece per table) loves this aspect of it. The downside is that since it has an AV value, some units (we shall say Wratihknights to keep the Eldar theme going) will try to just Blow Up the Bastion to let the Wave Serpents shoot the stuff behind it. Luckily AV14 isn't a gimme even with Str10 and it cannot be glanced to death.
The Emplacement options from the Aegis are there as well, so if you need a Comms Relay you can still bring one, and you can throw an Icarus or Quadgun on top as well. The size once again helps the Bastion here because it can let your gun with interceptor and any unit with them to have a very respectable sight of the board especially compared to the ground level Aegis. Personally I prefer the Icarus instead of the Quadgun because I like using units with high BS to man it, but to each his own, and there are many other places to find the math behind which is "better".
A not inconsequential benefit is that it also comes with 4 Heavy Bolters that can shoot automatically, at BS2 at the nearest enemy in LoS, while not amazing it is a couple extra shots at FMCs (for grounding checks) and infantry that could go from one lone model surviving( or being one kill short of forcing a morale test) and end up being a game winner.
The final benefit is hiding a small unit of cheap troops inside (henchmen, cultists, etc) and then having them exit turn 5 on top of a nearby objective. One of my favorite game highlights once included a unit of Wracks I had on top jumping off the battlements on to an Objective nearby (gotta love the FNP for the impact tests)
I hope this is helpful for those who were considering a Bastion