I would not let the user adjust the opacity level, but you can still disable RGBA in the gtkrc per-widget basis, just find out the widget you want to have opaque and add:
engine "murrine" { rgba = FALSE }

That can make my buttons opaque, but it won't make my editors translucent :-)

Yeah, a translucent editor is something totally strange, I would not go for it (you have to read the text, it must be opaque).

Yet, the most popular translucent app is the terminal... how is that different?

I would also like to be able to define the widget opacity in the gtkrc. There are a few little things that I would like to make semi-transparent that are not.

I, myself, when I was messing with rgba apps using an rgba module, changed some of the constants defining how opaque certain parts would be, the ones in support.h. Though of course, that meant recompiling murrine every time I made any kind of change. It also kills theme customizability. Everyone has to use the opacity levels you decided on, unless they recompile the engine. I imagine it would be a huge pain to allow opacity customizations on a per-widget basis. But you already use constants, which can be changed in the source easily enough... why not allow them to be changed in the gtkrc? What if some themer would like every widget in his theme to be opaque but menus and toolbars? What if some themer just wants the backs of his windows to be translucent? What about any other little tweaks, to make it as transucent as he sees fit? Right now, he'd have to distribute a modified version of murrine with his theme to accomplish it, which is far less than ideal. So why not change those constants you have in support.h so that they can be defined in the gtkrc?

I also vote for being able to set the opacity of widgets in gtkrc. I added rgba support to xfce-panel and you could image that I want my panel be more transparent than my other widgets. It would be very nice if we can configure that in gtkrc.

Why we cannot adjust the opacity level?
WHY WHY WHY?

I too would also like to see the opacity options in gtkrc. Rebuild the engine every time you want to make an opacity change?? Puh-lease Murrine is definitely falling short of what it could be.