You know, I popped in to this thread the other day --briefly-- and only came back today to see if I had actually seen it here and not just imagined it.
Are we really discussing this? THIS, of ALL things is a point of contention? REALLY? REALLY?
This is really a lot more relevant for discussion than a lot of threads which recieve a huge number of responses. In general trek discussion there tends to be some basically ridiculous threads, that, while fun to speculate on, are not really as relevant as Kira's hair. A few examples of recent threads ive seen or commented on are things like: 'Why dont they target the bridge?', 'Why are they always the only ship in range?', 'How do teleporters operate in civilian life?'
Like I say, these are fun to speculate on, but realistically they arent that relevant, because Star Trek is a tv show, and a lot of elements exist for dramatic purposes. Obviously the bridge would be targeted, but it wouldnt be too great within the context of the show because thats where most of the drama takes place, and where most of the characters are. The 'only ship in range' bit is for obvious dramatic purposes. The teleporters were only ever included in Star Trek because they couldnt afford the shot of a shuttle landing, they allow the trek characters to move about without having to worry overly about the inworld logistics, or real world production costs.
On the other hand Kira's hair is actually pretty important. In a world where most characters have no choice in what they wear, the choice of how they keep their hair is very important, because its the only way the audience sees how that character likes to present themselves. This is a very personal choice, it is a presentation of that persons personality. This becomes even more important in a show like Deep Space Nine, which is so much about character.
I actually quite liked Kira's hair in season 7. On the surface Kira doesnt seem like the sort of person who would bother with it; shes tough, she doesnt like fussing around and shes certainly not a girly-girl. However, look a bit deeper Kira does have this sort of fun, impulsive streak, and I think it actually fits for her to occasionally just do something different with the way she looks, just for the hell of it. The girls I have known who changed styles the most are really more tom boy than girly-girl, but its just something fun for them. A girly-girl puts loads of fuss into their appearance because they value it, girls like Kira mess around with their appearance just for a change, because they dont take their appearance that seriously. Things like this, which arent immediately obvious, give characters a lot of depth. You might compare it to the friendship between Bashir and O'brien, on paper it doesnt really seem to fit, its not something which immediately seems right, but in actuality it works well.
The point I was making is that the actress playing the character suggested a change of this kind as a meaningful expression of Kira's increased comfort level and increased awareness of her importance as a public figure (or words to that effect, probably somewhere in the DS9 companion or some such thing). 
