What do I get if I install a custom ROM instead of installing new launcher, new phone app, ...?
Two major cons to simply using a different launcher Vs. a whole new ROM:


Custom ROMs often offer a later version of Android than is available for your device from a manufacturer or carrier.  No amount of launchers will give you the newer functionality of latest OS versions.
Very often removing manufacturer/carrier "bloat" is not as easy as deleting some APKs from /system/app directory.  In cases of custom UIs such as Samsung's TouchWiz, or Motorola's BLUR, or HTC's Sense, many of their apps which replace stock core functions (e.g. Phone/Dialer, Contacts, etc.) are interwoven with the system and each other in such a way that simply removing them may cause crashes or other system instabilities.  Installing 3rd-party replacements does not resolve anything, since these apps are always running in the background, using up valuable resources.