From what i understand true "bookshelf" speakers are designed so that when you place them on said bookshelf they will not sound boomy because of their close proximity to solid surfaces. if you were to place them on stands i believe they would loose an awful lot of their bass sound - although you could counter this if you placed them so there was a wall very close behind them. Conversley i you took a proper "standmount" speaker and placed it in an actual book shelf it would result in a very boomy sound.
the name bookshelf is just the term used to describe them only as they will always sound better on stands. the stands give them stabilty which help with bass reproduction. all speakers sound better on stands but thats not to say you have to use them i guess its up to you