Certainly. And if there is a big bad there, pour EVERYTHING at him. But if we're talking about an everyday campaign set of encounters, then blowing your 9th level slot quicky just to one-shot that purple worm (etc), and only be able to get it back over tactful short rests trading warlock slots...
What level do you want?
My point would still be the same, if you're planning on shooting a 9th level Scorching Ray and quickening an EB you're still burning resources at a very high rate.
At level 20:
Scorching Ray lvl9 + EB + Hex + HexCurse + 4xAgonizing Blast =10d6+4d10+14d6+84+20 = 210...
Assuming you can't, and the quicken is to put out an EB:
At level 5 (practical):
Scorching Ray + EB + Hex + HexCurse + 2xAgonizing Blast = 6d6+2d10+5d6+15+10 = 74/75
vs
EB x2 + Hex + HexCurse + 4xAgonizing Blast = 4d10+4d6+12+20 = 68
The main issue is you're spending spell points AND spell...
Can Scorching Ray be quickened to two casts, though? Doesn't that break the "You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action" rule in PHB p.202
What about sacrificing a 9th level slot and going Hexblade 5 / Divine Soul 15 - that way you can pick up the extra invocation, keeping both Devil's Sight and Book of Ancient Secrets?
This seems to be the best compromise for characters not expecting to reach level 20. Thoughts?
Another minor one for the 2.28 beta -> Hexblade patron does not give proficiency with martial weapons as it should under the Hex Warrior entry on p.55 in Xanathar's.