Formations and special detachments - good or bad?

A topic that I'm sure has been discussed before, but I'm just getting into seriously considering using formations and alternative detachments.  My armies were pretty much built in the olden days of one force organization chart to rule them all, and even newer ones have been built with that mindset.  I had considered a Khorne Daemonkin army using their special detachment and formations before deciding to just do more regular CSM instead.  I have some other ideas floating around that may fit better with some of the special arrangements out there than a regular combined arms detachment, so I have been thinking more about the idea behind current ones.

I'm conflicted about if these alternate detachments and formations are good for the game and variety or not.  On one hand, I really like the idea of different factions organizing their forces in different ways, and we've had that in the past sometimes with a sub-group getting +1 Fast Attack and -1 Heavy Support for example, or the method of a particular character or type of character allowing something to count as Troops.  And it's neat to be able to do a specialized force similar to what we often see in the background without having to be as worried about filling out mandatory slots that don't fit with the theme or running into slot limits quickly.  I like the idea of being able to do a Wind Rider host, an all-terminator force, an Ork army full of walkers and meks (although I am a bit puzzled that some of the most obvious ones like a Space Marine Assault Company or Devastator Company haven't been done yet).  That's awesome.

But these don't just let you take things in a different arrangement.  They give you extra rules and sometimes even free things, like free transports or upgrades.  With that, they may move from interesting way to build your army to "why take those units any other way"?  If you're leaning towards something that is close to one of the alternative ways, why not go a little further and pick up those free rules and upgrades?  Could this actually end up limiting the variety and creativity we see in armies, with endless similar Skyhammers or Aspect Hosts?  We're also seeing more things being able to be taken in squadrons, with special rules for spamming the same thing.

However, even the "regular" detachment, the Combined Arms Detachment, gives you free bonuses.  Objective Secured Troops, and able to re-roll Warlord Traits from your own plus any of the rulebook tables. 

Formations and different Detachments are probably here to stay in 40K for the forseeable future.  What are your thoughts?  Are they great?  Bad for the game?  Good idea but should have points costs?  Good idea but should have more effort to balance their benefits vs. other options?