It'd make it confusing more than anything else.
One day I could say,
"Ohhey, Zozane008 posted here!"
Respond to her.
Then the next day,
"Wait... why did Kukuraba respond to me?"

umm...but what if people around the site (If you are in the sb, forums, ect... a lot) know you with one username...you wouldnt want to change it and go "Oh I used to be so-and-so, but now I am so-and-so"

It would also make it more difficult on staff.  Like; if they are after one user for breaking the rules, they could change their username and deny the fact.

I'm sorry, no support D:

I agree with Loki and Fun.  Sorry, no support. :/

And, like what Fun said, people who are well-known around this site wouldn't want to change their username.  And another problem could be if someone changed their username to something else, and another user took their previous username.  There would be a lot of confusion. ;c

Example-

Graveyardfox- *changes username to sPoNgEbOb12345*

Zozane008- *changes username to Graveyardfox, sPoNgEbOb12345's previous username*

saryapink- Hello, Loki! :D

Graveyardfox (Zozane008)- Hello, Sarya! :D

sPoNgEbOb12345 (Graveyardfox)- Hello, Sarya.

saryapink- Um, I'm talking to Loki, not you, Spongebob. o_o;

sPoNgEbOb12345 (Graveyardfox)- I'm Loki. xD  I just changed my username.

Graveyardfox (Zozane008)- No, I'm Loki. D: *is trying to trick Sarya*

saryapink- Um.... o_o;

sPoNgEbOb12345 (Graveyardfox)- I'm seriously Loki.  Please, whoever you actually are, stop pretending to be me. :/

Graveyardfox (Zozane008)- Fine, I admit it, I'm not Loki.

saryapink- ...... What the heck is going on here, seriously...? o_o;  Who's really Loki?

sPoNgEbOb12345 (Graveyardfox)- I am.

saryapink- Then who's that...? D:

Graveyardfox (Zozane008)- Zozane008! 8D

I think I made that 'example' too long and confusing. D8  That always happens.  And I know you probably won't ever do that, Zozo (or will you? xD), and Loki wouldn't ever change his username to something like that. 

Zozo so would. xD
I actually like my username though. o3o But if I did, I'd make it something bada.ss. :P

Zozo would.

Lul. Loki-Zozo.

No support, Sorry

And it sort of bothers me to see you using chat speak, Please don't

There has been talk of an item to change username. If this does happen, it will likely be a credit shop item, and it will definitely be something that is more than just a "click and use" item, but a process. Approval for the name change would have to be submitted to staff for review, and a record of the former username would be permanently fixed to the user's profile. The old username would also remain unavailable.

This is only a "what if" though - nothing is concrete, and there may not ever be a release of an item like this.