[ktizo]

You can install software while updating your system, just not by using the same process that is currently updating your system and there is no particular distinction between software and system anyway.

A better apt queuing system might be nice admittedly, but if apt is installing software/updates with dependencies, then letting it complete before other software with their own dependencies are added, might be a good idea.

But you can always use one of the other myriad of ways to install software other than apt, while apt is running.