Why do modern distros use startups linked from rcx.d ?
 

Just wondering...is it so that you can easily enable/disable the startups without deleting or renaming the script?

Nope, because all you'd NEED to do is chmod -x'ing it ...
I think (I don't know, and Slackware doesn't actively
use that style [while it does support it]) it's because
it makes it easy to decide in a) which order and b) for
which runlevels to execute a script.

Tinkster correct me if I am wrong but it also deals with the fact that a script only needs to be written once then linked too. Most of the rcX.d scripts are symbolic links. With that and the ability to decide on what will be called on each runlevel. That is how I always understood it. That was one of the biggest differences in BSD style in sysV systems I think.

Nope, that's perfectly correct. :}

The wrapper that loops over the scripts (symlinks) in
the per-runlevel directories will invoke the with a
start or stop, depending on their name (K vs S).