Ordinance drops take into account kill assists, close calls, and a variety of other things as well. Once you get to a certain amount of points, you get an ordinance drop. The current things included in ordinance drops are overshields, damage boosts, speed boosts, the scattergun, and the Sticky Grenade (the grenade launcher that looks like a pistol). While these will give you a decent advantage, when you use an ordinance drop, it doesn't always land right at your feet. All ordinance drops also have a waypoint above them for every player on the map. So, if you are trying to hide, or are in a lower level, calling in ordo is a bad idea because it will either give away your position, or land above you.
Now, let's get onto the OP. In this theoretical situation you've crafted to point out that this is a bad idea, it seems as if you are talking about a 1v1 while playing Infinity Slayer. An experience player vs. an inexperienced player. Now, in this situation the inexperienced player is ALWAYS at a disadvantage, unless you are playing a game where skill doesn't matter at all. However, if the inexperienced player can manage to get five kills (mind you, not five kills in a row without dying, but five kills AT ALL) then they will get an ordinance drop for their troubles. And they can use it whenever they want to, it doesn't go away unless they use it.
Historically speaking, Halo has always had a pretty steep learning curve when compared to other leading FPS games. However, in every FPS game, an inexperienced player will not commonly beat an experienced one. That being said, you point is not valid because you used something that is applicable to every feature of every FPS game invented so far. If we took your theory of it giving a bonus to the experienced player meaning it's a bad idea and applied it to other features, then customizable controls are a bad idea, maps are a bad idea, power weapons and vehicles are a bad idea, and precision weapons are also a bad idea. Basically, entire games become a bad idea. This renders your argument about inexperienced players vs. experienced players invalid.
So, let's talk about what would happen if we were to pit two experienced players of similar skill level together. What would the ordo drops be then? They wouldn't give either player an advantage, because both can use them to their own benefit. They add in a bit of fun and thrill instead. It adds in a little bit of strategy, rather than just running and gunning.
A for instance of the strategy that ordinance drops will bring to the table:
You get an ordinance drop because you've killed five enemies, however three of the five enemies have respawned near you. Calling in the ordinance drop will alert them to your location and give them a chance to kill you for it. What you decide to do instead is run back to where your team is, and allow the guy who is obviously having an off game pick it up instead to help your teams chances of victory.
As you can obviously tell, I do not consider Ordinance Drops to be a bad idea, and I fail to see how you could think that they would be.