
In the case of Munda's Vanguard, by tapping itself and another untapped Ally you control, you get to put a +1/+1 counter on each creature you control. That's quite a power boost if you happen to have a lot of creatures on the board when you use it. While a 3/3 for 4W isn't too exciting as a "boss" monster that doesn't immediately affect the board, he does give a counter to himself when his ability is used. He's functional, if not exciting.
This deck does lack the overall "bang for your buck" that the Twisted Reality deck has, due to having 10 copies of the new Colorless Basic Land Wastes contained within it. But as an introduction to some of the new Ally cards in Oath of the Gatewatch, it seems to do the job.
Creature (26)
1 Cliffside Lookout
2 Expedition Envoy
2 Kor Castigator
2 Ondu War Cleric
2 Serene Steward
2 Kalastria Healer
2 Kor Scythemaster
2 Vampire Envoy
1 Drana's Emissary
2 Spawnbinder Mage
1 Drana's Chosen
2 Zulaport Chainmage
1 Cliffhaven Vampire
2 Kor Entanglers
1 Munda's Vanguard
1 Malakir Soothsayer
Non Creature Spells (9)
2 Allied Reinforcements
1 Dutiful Return
1 Dazzling Reflection
2 Gideon's Reproach
2 Tar Snare
1 Smite the Monstrous
Land (25)
1 Blighted Fen
1 Evolving Wilds
14 Plains
9 Swamp

The other Cohort abilities are not overwhelmingly powerful, but do offer incremental advantages over time. Ondu War Cleric's Cohort ability grants you 2 life - which on its own is rather mediocre, but it does interact well with the life-gain & drain subtheme in this deck that we'll get to in a little bit. Spawnbinder Mage's Cohort ability lets you tap down a target creature, useful in making sure your opponent doesn't stone-wall your attacks. She is a bit under-powered, though, as a 2/4 for 3W (4 mana).
Zulaport Chainmage is a 4/2 for 3B, which is relatively aggressive, plus he has a Cohort ability to make an opponent lose 2 life. The best of the non-rare Cohort cards, though, is probably Malakir Soothsayer. Her Cohort ability makes you lose 1 life but also draws you a card. Card advantage is always welcome and her 4/4 body on a 5-mana (4B) creature is solid.

There are a number of other ways to gain life in the deck, such as Vampire Envoy, which gains you 1 life whenever he is tapped. Serene Steward also benefits you whenever you gain life, by allowing you to place a +1/+1 counter on a target creature for a single White mana. Unfortunately, the most obvious way to consistently gain life, creatures with lifelink, isn't included in the deck.

The "man land" Shambling Vent is a white/black dual land that can become a 2/3 creature with Lifelink until end of turn for 1WB. It's already a very popular land that's proven in competitive play, so having a playset will both fix your mana and give you another source of lifelink. Also, when they become creatures, they benefit from any abilities that affect creatures. You'll want to activate the Vent before using Munda's Vanguard's ability, since the +1/+1 counter put on a land remains there even after it reverts back to just being a land.
If you're looking for another inexpensive way to fix your mana, Scoured Barrens is a common land that comes into play tapped, but gains you a life in the process. Ordinarily this incidental life-gain isn't worth playing these lands competitively, but in a deck that wants to consistently gain life, these are fine to draw in the late game. Ally Encampment is good to fix mana for casting your Allies, and in the late game you can even sacrifice it to get back an important Ally back to your hand from the graveyard.
Cohort seems like a decent ability for Limited play, but in Constructed, it seems a bit underwhelming. Allied Reinforcements is a nice way to get two 2/2 Ally tokens in a hurry, but it may be a bit slow, and you're not really playing many Rally abilities in here at present.
The best direction to go if you want to mold this into a competitive deck is to go more in the direction of Mardu Allies, by adding Red. But if you want to keep the spirit of this deck, you could focus on making the deck more aggressive by adding Drana, Liberator of Malakir and Hero of Goma Fada - whose Rally trigger makes your creatures indestructible. Obviously the very best card you can add to this deck is Gideon, Ally of Zendikar, a planeswalker so useful he sees play even outside of Ally decks.
If you're looking to try out Allies, this isn't a bad place to start. Cohort is an interesting ability, but it doesn't measure up to the usefulness of the Rally triggers from Battle for Zendikar. Adding Planar Outburst as a board-wipe and Ruinous Path & Stasis Snare as targeted removal could make this a solid enough deck. Black/White allies may be a fair deck in the future.