I've heard that Pact Magic is a level or two behind full caster using spell points as a conversion system. I intended to see if that was true.
Methodology: I took the number of spells a warlock can cast per day and converted them to spell points using the Variant rule in the DM. So, if a warlock gets two 3rd level spell slots, I took the cost of a third level spell (5) and doubled it. I then assumed a warlock would refresh all their spell slots on a short rest and did one short rest (doubling) and two short rests (tripling). I compared that then to what a half-caster would get as far as SP again using the DMG.
I did not account for Mystic Arcanum, the free Patron spell, or any invocations. This is pure Pact Mage to half-caster.
Results
Analysis:
Pact Magic without any rest is awful. It's almost worse than an EB/AT 1/3 caster.
Pact Magic with one rest is almost equal to half caster. Admittedly, it maintains a solid lead through most of the progression (finally falling behind only at 15th level) but the half-caster pulls within a point or so several times.
Pact Magic with two rests keeps pace with a full caster for the first ten levels, but after that falls bending hard. Topping out at the equivalent of a 13th level caster. Again, MA would make up a lot of slack on this (effectively emulating 43 additional points worth of high-level spells right around the same time a full caster gets them) but the limited nature of MA doesn't quite equal the full caster's range of high-level magic.
So, you could look at it like the warlock is effectively a third-caster, a half-caster, or not-quite a full caster, depending on the number of times you rest. But again, that doesn't quite tell the whole story. Even if a 10th level warlock has 28 spell points to play with after a rest, they are still tied up in only four uses. He can't burn only three points to cast Misty step, he must burn seven. So even as his point amount increases, the amount he must spend to do anything also rises. He has the power budget of other casters with not the versatility.
It appears WotC opted for the equivalent of a warlock +1 rest to equal a half-caster. It makes sense to pick one rest as two is not always possible (best case scenario) while no rests are too stingy. It also makes sense due to the class's slots topping out at 5th (with higher being handled by MA).
Final Thought: It's still wild to think that a warlock under 10th level is roughly the equivalent of an Arcane Trickster if he doesn't get a rest, a Ranger if he gets one rest, or a Bard if he gets two rests. It shows the wild disparity in power all based on if you can get your contracted breaks or not!
I think it also shows why people have such wildly different experiences with the class. If you were in a game with frequent short breaks, the warlock felt on par with the other main casters. If you were in a game with few or no short rests, you were playing with one hand tied behind your back.