Interesting suggestion. Maybe I'm wrong on this, but isn't that a really feeble spell, for 9th level? Let me explain my thinking: It's a 15 foot emanation. If they fail the Save they get Dazed. They take damage automatically, but not much. And there's nothing in the spell to keep them from simply walking out of the area. Even Dazed people can still take a 5 ft step, can't they? (Dazed gets no action, but 5 ft step is listed as "not an action" in the rules, IIRC.) Regardless of what the spell description says, the effective duration is a maximum of three rounds, the longest it could take to leave the area using 5 ft. steps. Wouldn't Tentacles do a better job of holding them?
No 5' steps like others said, but even if those were possible, three rounds of dazing equals death in high level play. This is not about keeping the opposition where they are, this is about denying them actions of any kind, including purely mental stuff, supernatural abilities, words of recall, what-have-you. Moreover, nothing in the books is immune to daze effects (barring extremely few, very niche, not-likely-to-be-known-by-DMs immunities that are, in fact, available). Undead, Constructs, Oozes... the works. Just take care around demons...
Also, the damage is actually quite good. It's untyped, and it's 1d6/CL per round. That adds up quickly.
Tentacles? Targets are
not unable to take actions, can make a check every round to escape, and lots of stuff is immune (or functionally immune due to high bonuses) to grapple attempts at the level we're talking about. Also, the damage is lousy. No comparison.
The reasons I'd still argue against Maw of Chaos are a) SR: Yes sucks, and b) this is a very good dual threat (blast/BC), but for a 9th level spell, likely one of only two the character will ever know, I'd excpect something more suited to telling the laws of physics to shut up, sit in a corner, and cry. You know, not just another combat tool, even if it's a very good one. Something a little more versatile, more comprehensive. Something capable of being used in near every situation where the party turns to you with that desperate, hopeful, puppydog, "And now what?!" look in their eyes.