The Chasm Guide is really more of a combo card than it is a stand-alone card. If you were using Rally the Ancestors or March from the Tomb the Chasm Guide would make a lot of sense because you'd be suddenly bringing in several allies all at once who would like to immediately attack. As is, the Chasm Guide is mostly just giving Haste to a Zulaport Cutthroat, and that doesn't really matter.

The Rally mechanic is that when it triggers it gives the bonus to all of your creatures. For something like Menace or Lifelink, that can be a really big deal. But haste literally only matters on the turn a creature comes into play, so if you aren't bringing out more than one creature at the same time, the fact that the Chasm Guide is giving haste to multiple creatures isn't helping you. You might as well save mana and use a Reckless Cohort (or more likely one of the cheap White allies), because without creature spam support you're basically using the Chasm Guide as a warm body with the Ally tag. Or get a bigger body with a Rally effect you'll use more like an Ondu Champion

Chasm Guide is to be used to combo with March from the Tomb in the Sideboard.

Four copies in the Mainboard are there to ensure it has the most chance for draw, to replace one after a board wipe, and to re-trigger the Haste Rally.

I understand what you're saying but it works really well with Menace, which is why there are four copies of both Creatures with these Rally abilities.