6202740
it seems you are saying two different things:
Breaking a unicorn's horn does not disable their ability to use magic; it would be the equivalent of a human losing a fingernail and expecting them to be unable to grab things.
And:
It is the only appendage of all the races that can grow back naturally - an equivalent would be a human regrowing an entire limb.

Is it like a limb or is it almost entirely cosmetic?

They're all going to go to jail, aren't they?

6206886 A unicorn's horn assists in the casting of magic, but it is not as important as other races think.

The focus point - that is, the area where the unicorn utilizes magic consciously - is the centre of their forehead, not the horn itself. The horn however, functions in the same way a set of training wheels helps one learn how to ride a bike. This is because a unicorn can see the tip of their horn and by focusing on it, they have an easier time of using magic. With practise and time, unicorns eventually learn how to use magic without the assistance of their horn.

Even so, a unicorn's horn is dear to them; it is strong enough to be used as an emergency weapon, it holds a place among unicorn culture as a symbol of one's sense of hygiene, and - though this line of thinking has largely died out in present times - it serves as a reminder of how superior unicorns are to their lesser brethren.

Also, despite what many think, a unicorn's horn is as much an erogenous zone to them as a hand is to a human - it largely depends on the individual's tastes.

6206989 Thanks for clarifying.

Discord is kind of the WMD of characters when it comes to conflicts with opposing sides, whoever gets him wins usually if the the other side doesn't have the elements of harmony or something of equal or greater strength to force a stalemate or instant win.
Therefore the moment Discord was mentioned to be on the side of Celestia and Luna the whole situation kind of lost tension unless NMM has something that can counter him in a manner that is not instant defeat otherwise hes kind of pointless to include in the first place.

6206989
But what evolutionary advantage does it have besides being sharp and pointy, then?

6207914 Old unicorn lore will tell you that it serves as a symbol of their superiority over the other races.

In truth, it really does have little else going for it other than being sharp and pointy and acting as a training tool for casting magic. Many non-unicorn scholars go so far as to say it is vestigial. Naturally, this claim is very controversial and guaranteed to incite anger in unicorns.

6210187
The difference is that the appendix used to have a use.

6211832 Yeah, old languages were never my forte; hope that change makes more sense.

I don’t Chrysalis would be eager to test Canterlot again

Missin' a word

7291475 Thanks for pointing that out.