I know what you mean, personly I'd prefer to have each set only once.
For some occasions theres just little of a faction if you have all sets of the theme even, like with the new redcoats for example, or the swan knights. In that case you kind of need extra soldiers. Some factions simply lack balance.
Still, I am not much into wars or such, I prefer the small scale conflicts so my soldiers are usually just used for guard duties.
