It's difficult to address this question without beginning with Samuel Johnson's Preface to Shakespeare, in which he tackles this very issue. In the eighteenth century, in which Johnson was writing, there was an emphasis on Classical forms, such as Aristotle's unities of place, time, and action. Theses unities, which Molière adhered to, dictated that a play take place in one place, avoid subplots, and be restrained to a 24-hour period—or as in Molière's case, real time. Shakespeare, as the question notes, ignored these rules.

An advantage of observing the unities is that they impose a form or grid on a play that forces a playwright to stay focused. As with writing a sonnet of 14 lines, a rigid form keeps a playwright's writing concise and on track. It ensures a play is easy to follow. Its compression can also heighten the emotional impact of a story.

Shakespeare's ignoring of the unities, however, gave him an enormous amount of freedom to move his plays from place to place, such as from Rome to Egypt, introduce subplots to provide comic relief or illuminate a main plot point, or cover months or years of time. This freedom allowed for a vast amount of richness and creativity to enter his plays. And as Johnson notes, audiences realize they are watching a work of fiction and are able to adjust themselves to the world a play presents. They are not concerned about rigid realism for

The reflection that strikes the heart is not, that the evils before us are real evils, but that they are evils to which we ourselves may be exposed.

Further, Johnson believed Shakespeare's characters were so realistic that that feature of his drama imposed its own unity.

A drawback to the unities is that they can become a strait jacket in the hands of a lesser playwright than Molière, imposing artificiality on a play or forcing characters to have experiences that don't properly fit real time or one location. You don't, for example, want too much reporting of events that happened in other places, as that can make a play static and dull. Moliere is a great playwright, but he does not have the stature of Shakespeare, and that is in part because his plays are constrained in what they can do.

A drawback to freedom is the concept of "so much rope that you hang yourself." Shakespeare could carry off his complicated plots because of his consistent characters, but even he sometimes could fall too heavily into comic subplots that threatened to turn too much attention from the main story. In a lesser playwright, the lack rules could result in a confusing mess of a drama.