but if you use order independent transparency, why not shade them forward? then you just store the color (and sometimes depth) per layer, if you want to shade transparent objects the deferred way, you have to write out a gbuffer for every layer, this could be quite expensive.

In my opinion, for transparent passes, one of the techs described here is the way to go

I put together a hybrid method, rendering transparent/alpha blended materials in a separate pass(es) using deferred shading. It's a bit more flexible than a using forward rendering and has the bonus of unifying the lighting between the opaque and transparent steps. As for order-independance...ShaderX7 describes an interlacing method for deferred rendering transparency; it could be used to do OIT but is very limiting on the number of per-pixel transparent layers.

Rendering using OIT could be possible in forward rendering, but that defeats the purpose of the OIT, doesn't it?.

not really, OIT solves the problem of the proper order of alpha fragments/pixels. it's needed for proper transparency even without any lighting. check out

I think the only shading comes from a fresnel term for the blend intensity.

the solution you want to choose is very dependant on what you're actually rendering. having just one layer of glass (e.g. window) might work with simple MSAA and a custom sample mask where you writ every second pixel in case of transparency. it's simple to implement, fits naturally into the deferred shading pipeline with all the other effects. particles on the other side won't work with satisfying results with this solution. but particles can be shaded per vertex (or at least you can select like the 4 most contributing light sources per particle/vertex and use those in the usually forward rendering fashion (but that won't work for big windows that might be affected by several tiny lightsources. For tiny objects (e.g. a bottle), you might get away with cubemaps only that have lighting backed in etc.

And lighting + transparent objects are not the only issues you might have, you also want fog (even fog areas?) to work fine, mirrors (e.g. a water plane)? you might want to have shadows + projectors, decals?

and on top, you need to be very careful regarding performance, while you have an exact limit of solid pixels on the screen (-> your resolution), transparent objects can easily add 10x the cost. you might become limited to e.g. the fillrate, which isn't that common in solid deferred rendering. you might also process a lot of fully transparent or overwritten pixel. that's the main reason why most games nowadays have very few transparent objects.