However, I don't understand why it is not advised. If you have contribs installed and the contribs repo enabled and visible, won't new contribs updates be included in the cron check for updates and notify you of them?
Yes, but...
Then you can chose to install or not.
That depens, if you use the default configuration and server-manager, you do not have the choice which packages to install, it is either all or nothing then.
If smecontribs does not show up on the software install panel then no upgrade notifications are sent. At least it seems to work that way on the upgraded 7.3 to 7.4 server I am running. Please clarify if I am wrong.
True, but the risk is in the amount of review that is being done on contribs. All core changes are checked if they really fix the issue, without having nasty side effects, for contribs this process is sometimes done, but most of the times not.
The chance of installing a contrib that might damage or wreck your system is a lot higher than for core packages.