A speech is usually something that has been rehearsed. This is because if you ask everyone to stop what they are doing and listen to a speech, it is usually expected that you will have something to say that's worth listening to - and the best way to make sure of that is to spend some time preparing what you are going to say and if possible testing it on some people before you try it on your 'real' audience. So the disadvantages of impromptu speeches are quite clear - you may not be able to come up with any ideas in a hurry, you will probably repeat yourself and also you will be too busy trying to think of things to say to present your speech in a clear and interesting way.
However, that doesn't mean that it is always a bad idea to make a speech on the spur of the moment or that speeches that haven't been prepared are always bad ones. There are many occasions where what matters is to hear what people think or feel about what is happening right there - or where it is important to react quickly to a situation.
There are some situations where an impromptu speech could be the best thing to do. This can happen at public events such as weddings, for example. I can remember one wedding where we heard all the usual speeches from the father of the bride, best man, groom etc - which were quite dull and predictable, carefully prepared with the aid of a handbook but not individual at all. Then the bride jumped on a table and with no thought at all just gave a little speech thanking everyone for being there and saying what a wonderful day it was etc - it just made the whole wedding more warm and special. (I'm sure she'd had a few drinks, but never mind!)
In other situations, such as public meetings, it's very valuable if you can speak at short notice, because otherwise the chance to give your views or contribute your ideas will be lost. In these cases it's better to stand up and say something, even if you don't put it very well, than for your points not to be made at all.
So: Overall, the more you prepare your speech the better it will be, but there are times when spontaneity is a good thing.
However, that doesn't mean that it is always a bad idea to make a speech on the spur of the moment or that speeches that haven't been prepared are always bad ones. There are many occasions where what matters is to hear what people think or feel about what is happening right there - or where it is important to react quickly to a situation.
There are some situations where an impromptu speech could be the best thing to do. This can happen at public events such as weddings, for example. I can remember one wedding where we heard all the usual speeches from the father of the bride, best man, groom etc - which were quite dull and predictable, carefully prepared with the aid of a handbook but not individual at all. Then the bride jumped on a table and with no thought at all just gave a little speech thanking everyone for being there and saying what a wonderful day it was etc - it just made the whole wedding more warm and special. (I'm sure she'd had a few drinks, but never mind!)
In other situations, such as public meetings, it's very valuable if you can speak at short notice, because otherwise the chance to give your views or contribute your ideas will be lost. In these cases it's better to stand up and say something, even if you don't put it very well, than for your points not to be made at all.
So: Overall, the more you prepare your speech the better it will be, but there are times when spontaneity is a good thing.