If it was an absolute effect then why did Harry, when facing the entropy curse and the heartripper spell, didn't simply block them with a circle?
The entropy curse because a) a circle is far too easy to disrupt, b) there were too many potential targets to try to protect, and c) he needed to trace the spell back to its caster to actually solve the case. Also, based on the events of Small Favor, it's pretty clear hat there are practical limitations to the size of a circle--Harry couldn't make one big enough to protect the whole studio.
The heartripper spell because Harry didn't know he was a target until he felt the spell hit him, and it's hard to focus enough to draw a circle around yourself when your heart's exploding out of your chest. Also, again, sitting around in a magic circle doesn't solve the case.
If it was an absolute effect, then why do supernatural creatures that are not omnipotent but are still powerful (i.e. summoned demons) fight to escape or pass through circles and the caster of the circle feels the pressure? And remember that even with an elaborate circle, Kim Delaney was still eaten by the Loup Garou.
As others have mentioned, you're confusing
magic circles with
summoning/binding circles. A magic circle isolates the area inside from the magical energies outside. You can't summon a demon in a magic circle, because the energy it needs to maintain the ectoplasmic body it creates will be cut off--we've seen Harry exploit this on a few occasions, by drawing a magic circle around a summoned entity and cutting off its source of magic.
The loup-garou was bound (or was supposed to be bound) in a binding circle--three concentric layers of wards designed to block three different phenomena. Since the loup garou is a physical being and not wholly magical, putting it in a magic circle would be marginally less effective than putting it inside a large cardboard box.
The only time we see Harry use a magic circle to trap something is with Toot-toot: Harry draws a circle but doesn't close it, then lures Toot-toot in with a call to his True Name and a fresh pizza, and once Toot's inside he closes the circle. Since faeries, like Red Court vampires, have actual bodies and not ectoplasmic constructs, that's a trick that can work without dissolving Toot into goo.
If it was an absolute effect, then why use simplistic circles against small things, elaborate circles vs bigger things and ridiculously complicated stuff against, say, the Loup Garou, the Erlking or the Archive?
Again, you're confusing two different concepts. A magic circle is a magic circle, though some wizards go for more complexity than others as a matter of pomp and circumstance. What you're talking about is a binding spell, which the game recognizes as a type of ward. I've already talked about why the loup-garou is in no way hampered by a magic circle. The Erlking, being a faerie, presumably
could be caught in one the same way Toot-toot was, but since Harry was calling him with a summoning spell as opposed to luring him in while he was present in the physical world, presumably a magic circle would cut off that magic and end the spell, allowing him to leave. The Archive, as I recall, was bound inside a fairly ordinary magic circle on top of which were layered very powerful wards and bindings to keep her from trying anything. The laser-pentagram circle, like I said, was because there seems to be a practical size limitation on circles.
