It is deliberate, and has its good as well as bad points.
Yes it can slow the unit down in some situations, since the cohesiveness of a leaderless Warband is not as strong as one with a full-time Leader.
However it also has its good points, as in a situation where a model may be outside Command Distance. Simply nominate that first model to activate first that turn, thus becoming the Leader, and it's very difficult to have models in a leaderless Warband outside of Command Distance, since the Leader can change from turn to turn.
Leaderless Warband's can also shift direction faster than normal Warbands, if per se you nominate a figure on one corner of the Warband as Leader for one turn, then a member on another edge of the Warband as Leader for the next, and so on and so forth. Yes it does slow them down a bit, but at closer distances, this can be a very valuable tactical option.