These updates are to make the phone more secure and the owner to be more in control of his device (closed source= someone else in control, opensource= software code is open for owner to check it so there are no hidden commands and setups). So not making a CSSU update is to keep your device using more closed source software so you are less in control of your device and there are many security risks in old packages which have not got security updates by Nokia in 10 years.

But the choice is yours. Just wan't you to know thnat it seems that in this case your action (not want to install opensource community's security updates and to get rid of packages someone else is in control) contradicts your intention (have a secure phone with packages and software you know what is inside) .

You should then actually wait for Maemo Leste Os update to be ready before using N900 at al (but you won't trust that either I think)l. Before that - your wish about your N900 being secure is not true but a dream. Worst think to do is using N900 without updating it and thinking it is more secure in that state.

Why you think N900 would be more secure with manufacturers old, not updated, closed source stuff which you can't even check (by reading its code) what that software is doing? Or maybe this is new to you. Opensource updates are updates with stuff that everybody can check there are no hidden scripts of code in the software. So this CSSU update should be the thing which you definately wan't for you N900.