As a tank, your job is to anticipate damage patterns, not react to them after the fact. When you don't know a fight, then you're forced to play defensively and hold on to cooldowns because you don't know what happens next. As you learn the timings, you stop playing reactively, and you can work on being consistent. This is much more beneficial to your healers than taking unnecessary damage because you are holding cooldowns in reserve, and then blowing them all in a panic when your health is in the double digits. There are no surprises when you have everything choreographed in advance.

You can think of tank stance as an on-demand cooldown. The main cost is the damage penalty. If you're in tank stance, it's either because you need the enmity generation, or it's because you need the mitigation. Your dps is directly dependent on how low your stance uptime is.

If the boss is just using auto-attacks, there's usually no reason for you to be in stance. Moderate damage attacks and cleaves, however, are often another story. Once you've established your cooldown rotation for the tankbuster set, you can put your unused cooldowns to use in smoothing out the rest of the damage from these cleaves, or to troubleshoot areas where your healers are under more pressure. This allows you to stay out of stance for longer. Short recast cooldowns like Sheltron can be extremely powerful in this regard, once you've memorised the cleave timings. You can think of this as bridging together two low damage sections of the fight, bypassing the need to swap back into stance in-between.