Bar ends are typically banned from group/club rides because of the potential to hook another rider when riding in a tight group. Because a flat MTB-style handlebar is wider than a drop bar, adding bar ends increases the likelihood of snagging another rider. (drop bars still can cause crashes though, which I've seen first hand)
On the plus side, adding bar ends gives you more hand positions to choose from (similar to using drop bars). This is very important on long rides (e.g. touring) since your hands can go numb and possibly suffer nerve damage if you keep them in one position for too long. Some people find the hand position on bar ends more comfortable too.
Papuass has been injured by a bar end in a crash. My experience is the opposite: the bar end protected my hand from sliding along the pavement at high speed, saving me a fair amount of skin. So it can go both ways.
If you're not doing group rides where bar ends are banned, I would definitely consider trying them for the sake of having more hand positions.