Only if added in quantities above about 25% of fermentables, and only when not inverted by acids. If sugars are boiled in the wort as they (usually) should be, they'll invert and the cidery taste won't be present. I've made strongs with 20% white sugar and with no problems at all.
As for molasses, I love it in stouts and porters. Be advised, though, that I've found it necessary to age them longer, as a youngish beer with any significant amount of molasses has a fairly assertive flavor, in my opinion. (I used dark, but not blackstrap, molasses).
I believe it is to add more fermentable sugars to the wort. Thus, raising the alcohol content, without adding body. It could actually thin out the body of the brew, and that can be desirable in heavily malted beers like RIS, porters, etc.
p.s. some info on molasses and muscovado. I just started using muscovado sugar in my heavy ales, porters & stouts. It carries the molasses and brown sugar flavor as well as some extra minerals...