You'll always have some fixity (unless you use a true pin, but that's not likely). The analysis assuming a theoretical pin forces the design of the beam, column, and connection between the two to be stiff enough to carry the load alone. Because of that, the relative rotational stiffness of the base tends to be very low if you follow normal detailing practice for a non-eccentric load base plate.
None of our analyses are exact, but we get as close as we need to.
Also, it allows for conservative sizing of the beams and columns. Assuming pin bases results in larger beam and column sizing to keep drift to a minimum. Then you provide some partial fixity at the base, and you end up with a frame that outperforms the requirements.
It is not easy to make a true pin. There are several things that affect fixity.
[ul]
[li]Anchor bolt location (Anchor bolts on each side of a flange act more fixed than anchor bolts near the center of the web)[/li]
[li]Thickness of the baseplate (thicker plates act more fixed)[/li]
[li]The foundation the column connects to.[/li]
[li]The connection of the column to the baseplate.[/li]
[/ul]
