first of all, let me make a few things clear:
1. within my gaming group, i am known as the resident "character switcher", which is not necessarily a good thing
2. if a new character is brought in, that character usually starts at 2 levels lower than the highest party member
3. my DM and group have been very lenient and understanding by allowing me to switch characters so many times, sometimes, just for the sake of wanting to switch a character, no actually character death involved
speaking from a "character switcher" point-of-view, there are many reasons why someone would want to do this, some "purer" than others. unfortunately, most of the times, the reasons are either meta-gaming or munchikin-like, but there are times as well that there are totally valid reasons, which are defined by your gaming group or DM.
however, i do not entirely agree with not being allowed to switch to a new character if the current character dies.
1. in my opinion, one of the thrills of DnD is the fear of death. if you are not allowed to switch characters, that fear is not felt that greatly since you are almost guaranteed to be brought back with the same character.
2. if you are not entirely happy with your character, character death is one of the few ways to bring in a new character. this game is based on fun, and if you are stuck playing a character that is no longer fun for you, then what is the point? one of the few ways to bring in a new character is the death of the old one, but if that option is taken away, how else can you bring in a new character?
these are just a few reasons why i disagree.
however, there are many reasons why frivilous character switching shouldn't be allowed, but, in my opinion, if a character dies, they should be given the option to choose a new character. i feel that unless your character dies, there is no other way to switch to a new character.
of course, your DM has the final say.
on another note, my gaming group went through the same situation, except i was the player always switching characters. my DM spoke to me outside the game session and told me his concerns regarding my switching characters throughout the campaign.
in the end, i think we both agreed that the ultimate goal for our gaming group is fun. although he would prefer i stay with my current character should i die, he would not disallow it if i was no longer having fun with my current character.
however, i also realize the amount of work he puts into the campaign and the amount of concessions he has made to me in the past. that being the case, i will try to take all that into account should my character happen to die. i understand that there is a difference between switching characters for legitimate reasons and for more frivilous ones.
however, depending on what is legitimate and what is frivilous is depends on who you are asking.
this is just my humble opinion, but i hope it helps you determine on how you want to handle your campaign.
1. within my gaming group, i am known as the resident "character switcher", which is not necessarily a good thing
2. if a new character is brought in, that character usually starts at 2 levels lower than the highest party member
3. my DM and group have been very lenient and understanding by allowing me to switch characters so many times, sometimes, just for the sake of wanting to switch a character, no actually character death involved
speaking from a "character switcher" point-of-view, there are many reasons why someone would want to do this, some "purer" than others. unfortunately, most of the times, the reasons are either meta-gaming or munchikin-like, but there are times as well that there are totally valid reasons, which are defined by your gaming group or DM.
however, i do not entirely agree with not being allowed to switch to a new character if the current character dies.
1. in my opinion, one of the thrills of DnD is the fear of death. if you are not allowed to switch characters, that fear is not felt that greatly since you are almost guaranteed to be brought back with the same character.
2. if you are not entirely happy with your character, character death is one of the few ways to bring in a new character. this game is based on fun, and if you are stuck playing a character that is no longer fun for you, then what is the point? one of the few ways to bring in a new character is the death of the old one, but if that option is taken away, how else can you bring in a new character?
these are just a few reasons why i disagree.
however, there are many reasons why frivilous character switching shouldn't be allowed, but, in my opinion, if a character dies, they should be given the option to choose a new character. i feel that unless your character dies, there is no other way to switch to a new character.
of course, your DM has the final say.

on another note, my gaming group went through the same situation, except i was the player always switching characters. my DM spoke to me outside the game session and told me his concerns regarding my switching characters throughout the campaign.
in the end, i think we both agreed that the ultimate goal for our gaming group is fun. although he would prefer i stay with my current character should i die, he would not disallow it if i was no longer having fun with my current character.
however, i also realize the amount of work he puts into the campaign and the amount of concessions he has made to me in the past. that being the case, i will try to take all that into account should my character happen to die. i understand that there is a difference between switching characters for legitimate reasons and for more frivilous ones.
however, depending on what is legitimate and what is frivilous is depends on who you are asking.

this is just my humble opinion, but i hope it helps you determine on how you want to handle your campaign.