I use the 16-35mm L for wide angle. Do i need wider, is there wider for indoors and landscapes?
I'm just getting back into photography, so I'm no expert by any means...but I've been doing quite a bit of research over the last 3 weeks and I think maybe some of my thoughts might give you some sort of answer to your questions...
First, there's definitely wider angles than 16mm. The popular Canon len is the 10-22mm. A couple posts up I mentioned a fast Tokina 11-16 f2.8 that came out last year. This one is stated on one review site (photozone.de) to be the best quality super wide angle out there...bar none....and this lens runs less than $600. So yeah there are definitely options. Another option is a 12 - 24 f4.0 Tokina. Also highly rated and low price, but a bit slower.
You might ask, well it doesn't seem like there's much difference between the low end of your 16-35 and the 11 or 12 low end of these others...I guess that depends on what you want out of the lens. I've read where on the low end, small differences in focal length can make a fairly significant difference in the field of view.
Referring back to the question you posed...whether you NEED wider is another story. Seems to me that it's personal preference, the type of shooting you do, etc. Do you often feel limited by the field of view that you shoot in? Obviously, when outside say shooting a tall building, you can theoretically keep moving farther away to adjust your field of view. However, inside a room, if you're as far away as you can get, the only option then is a shorter focal length lens.
Something to consider is that of course, the wider you go, the more you tend to have to deal with distortions like edge falloff and (what's it called) where the straight lines curve because of lens distortion.
As I recall, back in the day when I shot with my 28mm (which again is 17mm or so on a crop body) image distortion didn't become all that much of an issue. IOW, it wasn't totally obvious that the image had been shot with a wide angle lens. It's only with the super wide angle lenses that distortions become a major distraction...at least to me they are.
Don't know if that helps...this discussion is probably below your level of knowledge, but like I said, for me it's all fresh on my mind because these are the things I've been considering recently in making some decisions about what my lens aresenal is gonna consist of.
Take care.