I assume that the desired alias may not be the same as the CallerName in some cases. It may be, it may not be; hence why I indicated the alias has to mapped in some manner to know the destination. The alias field as you suggest would handle that.
Additionally, while it would be nice to dial “ck” to reach 1000, I am willing to bet that I can dial 1000 faster than you can enter the menu and then select alpha numeric and then manually input ck and the remainder of the domain/IP (granted the domain/IP may be defaulted to the same as the extension SIP URI that would normally accompany the use of the extension number). At some point the alias names may become many, so that as it is with remembering numbers, one may not remember them all and still need to refer to a directory. Additionally, using a nice easy two character alias is a whole lot different than remembering whole names such as Astylos and the issues that arise if there happens to be more than one Astylos, which may also encourage the need for a directory.
However, after having said all of the above, I would like to see it as well, but for a different reason. I have some clients that for whatever reason may have 5 digit extension numbers. Normally, these were a holdover from a prior PBX and rather than change them and the directories, they elected to keep same even though they have no where near the allowable extensions that 5 digits allow. On other systems, I have been able to take the 5 digit extension and use the alias to make it 3 digits as most are unique in that the last 3 digits of the 5 digit extension are unique. Folks can use either the 3 or 5 digits method.
You can/should submit a feature request for the function.