In the spirit of my last post. I'd like to be the only person defending Disenchant in this thread.
I've been playing a lot of White Eldrazi lately, and made a lot of builds for different expected metagames. While I tend not to run either, I've ended up putting Disenchant in some builds, but Fragmentize in none.
The specific reason I've done this is because of Car Shops. Car Shops is a matchup I've spent a lot of time working on, and in my experience, the instant-speed nature of Disenchant has been more relevant than the cheaper cost of Fragmentize.
Disenchant as an instant can lead to tempo blowouts by targeting a vehicle after it's been crewed, or the target of an Arcbound Ravager's Modular tokens on the stack. Disenchant can answer a Fleetwheel Cruiser or an Umezawa's Jitte before it connects once, which Fragmentize can't.
Of less importance, but still notable, there are a few relevant artifacts in this matchup with CMC > 4 (Skyship Skysovereign, Karn, Silver Golem, Precursor Golem, Wurmcoil Engine)
Further, the extra mana isn't near as much as a pain point for White Eldrazi as it might be for other decks - WEldrazi has a huge mana count, often some number of lock pieces are removed from both decks post board, you often end up with unused mana at the end of the turn, and this is not an effect you're trying to get to land on turn 1 or 2.
This alone isn't even enough context to make this decision though, because if you've noticed, Path to Exile gives you all the upside of being an Instant (for the cards we care about in this matchup), at CMC 1. The only reason I would run Fragmentize/Disenchant over Path to Exile is if there were some noncreature artifacts/enchantments I was worried about in other matchups.
In this case, I like Disenchant over Path to Exile right now because of the up-and-coming Stax-style decks, which have key cards like Ensnaring Bridge, Crucible of Worlds, Smokestack which can't be Path'd. While I might rather have Fragmentize than Disenchant against a Stax deck, Car Shops is so much more important a matchup, that Disenchant gets the nod. Besides, Disenchant has some edges in the Stax matchup too ... it works through Tangle Wire, it kills a Smokestack after they've lost a card to it, it kills a key spell the turn they draw it rather than having to wait and use your next turn's mana, etc.
This is all assuming I'm not running red mana in my list, which is usually where I end up these days. With red in the list, I sidestep the entire discussion and run a mix of Paths and Ingot Chewers, which are too good with Ancient Tomb and Eldrazi Displacer to pass up. If unrelated metagame factors caused me to dump red (like Karakas getting stronger or Mentor getting less popular), I'd likely be on Disenchants.
Disenchant is notably better against Time Vault/Key decks as well - not because of the Missteppability, but because a Sorcery is going to be too slow most of the time. This didn't factor into my decision at all, despite the fact that I might end up boarding them in.
If we're talking about different lists and different matchups (and I'm sure most of us are), there's probably a ton of reasons why Fragmentize is better instead.