@Masem
 
That syntax isn’t Mtg-specific. EVERY game with effect cards that stay in play uses the repeatable-cost-paying mechanic. Yugioh, L5R, Lord of The Rings, you name it.

@Masem  
The bolded Main Phase just means you can play it in your main phase, like they say on page 15 of the rulebook “Use Game Text”.
 
Or to quote the Comprehensive Rules: “(512.3) Cards with the timing phrase “[x] Phase” can be played or have their  
corresponding ability activated only to open a priority window during the  
specified Phase of their owner or controller’s turn. The turn player may  
choose to do this as another priority window closes, in which case a new  
window opens before the game moves forward.”
 
If it wasn’t repeatable, and worked only when you move into your main phase, it would say “At the Start of your Main Phase you may remove…”  
Like how Troublemakers refer to “at the start of your opponent’s Troublemaker Phase”, or Minuette refers to “at the end of your Troublemaker Phase”.

@Kazapsky  
However, I am fully aware of Enterplay’s card prose tends to be poor/confusing compared to how tight MTG gets it.

@Kazapsky  
The way I read this is that with “Main Phase” bolded like that is that once your turn moves into the Main Phase, you have that option to buff a friend. The wording should have been, if it was a repeatable use, “During your main phase, you may move a counter…”

@Masem
 
Unless you can only make one action per Main Phase, I beg to differ. SOP for wording card effects in any game is for once-per-turn effects to explicitly say “once per turn”.

@Background Pony #7BFA  
The wording on the card implies a once per turn activation.

@Masem  
Or let them build up and you can grant a huge buff all at once.

@Background Human  
Have both in play, and Derpy’s just tossing out +2 buffs each turn. Pretty effective combo

And that explains Equestrian Mailmare.