WHY SHOULD I UNLOCK MY AT&T CELL PHONE?

0. I travel overseas very often and want to use my cell phone in a foreign country. I can purchase a short-term SIM in that country and have it work instantly in my favorite cell phone.   Most foreign countries use the GSM cell technology like AT&T.
0. I was provided this phone by AT&T for little or no money.  I now have found the perfect phone for me, and it is unlocked, and will work on my AT&T network instantly.  I need to unlock my current phone and sell it to someone else.  I understand if I unlock it, it is more valuable since it can be used both on AT&T and on T-Mobile GSM networks.  Someone who is going overseas may buy it and take it with him.
0. I am having trouble loading and operating third-party and/or freeware on my locked AT&T cell phone.  I have discovered that sometimes AT&T prevents some functions from operating by locking them out.  It may be possible to use additional software by removing the AT&T lock on my SIM.  It is worth the money to get this freedom.
0. I'm not going overseas.  I am happy with AT&T and the software I have on my phone.  But it just makes me mad that a company dictates what I can and can't do with my property.  I am going to get my phone unlocked.  I may never sell it.  But it is mine, and is in my control.