The problem is, it will slow down the computer. SSDs tend to loose write speeds after the drive has been filled up. I don't mean completely full, but when all pages in the NAND have been used. The Samsun controller used in the rMBP solves this to an extent. However, it is not a good thing to have a full SSD or continuously keep writing to it. Also, NAND flash cells degrade with every write, so you have limited of cycles.
NAND cells will not slow down after they have been used. During idle time the drive controller cleans up these cells and performance is like new again. It is true NAND cells have a finite number or write cycles, but during any kind of normal usage that limit is not going to be hit for many many years.
Read the section in
this article about NAND lifetime for some calculations.
There is no reason to disable local backups with Time Machine just because one is using a SSD.
Local backups are managed by OS X and if the user needs the storage space the OS will shrink the local storage area. It is really a good feature that provides backups when away from a Time Machine destination.
If a user does not see the need for this feature that is fine, but having an SSD is no reason to turn it off.
If the feature is left on and for some reason one wants to erase the local backups, just turn Time Machine off then back on and the space is erased.