I have done a few 10 gallon batches (~12 gallons preboil on my system) with mine and have had no problems. If you put the bung in from the inside out, the liquid actually helps hold the bung in.
I would not use a wood bung for a boil kettle because of the possibility of infection coming from the wood and the difficulty in cleaning. Silicone, however, holds up to the heat and is easy to clean.
As to why people use straight sided kegs? Not sure there is one correct answer to that and I certainly can't speak for anyone. Looks is probably part of it. They are also less hassle since you don't have to add handles.
Personally, I have one of the older bung style, I have a coors style with it's slight barrel shape and a straight sided Bud style. They all work, they just fit my (in progress) brew rig slightly differently and that is nothing difficult to deal with.