I wouldn't go so far to say I
hate the insta-heal, but I do find it doesn't fit very well and personally I don't like using it.
I say it doesn't fit very well because it's consumable unlike all the other promotions which are a permanent bonus for the unit. It doesn't fit very well with the whole concept of "xp" (which I assume is generally short for "experience") being earned in order to learn new abilities. Yeah, I know a realism argument is weak in a highly abstracted game like civ but it's also important for game rules to be internally consistent with each other. Insta-heal stands as the odd one out from all the available options. How does it make sense that by getting experience from battle your warriors can heal to full health on just this one occasion?
Gameplay wise it makes for an interesting decision - the trade-off between a short-term advantage and a long-term advantage - so I can see perfectly why some people might appreciate it more than others.
I'm sure I'm not alone in my playstyle where I like to make highly experienced units (<-- hey look at that sweet Drill IV Phalanx

). This isn't something that happened recently - I was doing it sort of as far back as civ1 where you could get units to "veteran" status by having them win at least one battle or building them in a city with a barracks. Civ2 and Civ3 had similar mechanics, but Civ4 and Civ5 are where the lid was really blown open on this aspect of the gameplay, opening up vast and complex (relatively speaking!) promotion trees. Seriously it feels like there are more promotions in civ5 than there are units.
From a game enjoyment point of view, I think it makes a lot of sense that some players will try to achieve highly experienced units. They get unique abilities as a reward for you accomplishing some goal. This is one of the core things that would make the gameplay fun to players. Especially because in civ5 units are even more scarce (due to 1UPT and its consequences) to sacrifice a unit to never achieving its true potential by "wasting" a promotion just doesn't feel good even if the current tactical situation dictates it's the best action.
In short, while the insta-heal may make for an interesting option for the more tactically minded players, it doesn't seem very internally consistent against the other promotions and is unfavourable for players who take part in the unit leveling mini-game.