> why bother exactly if any given cmdline positional argument could be wrapped into " " or ' ' ?

Because action* is almost always more that one word. Because this is the only case a wrapped argument is needed. Its still not the nicest but it gives consistent results. Because fail2ban currently discarding arguments and not reporting so much as an error.

The testcase:
<pre>
./fail2ban-client -c config/ -s /tmp/f2b.sock -i start
add test pyinotify
set test addaction iptables
set test actionban iptables echo <ip> <cidr> >> /tmp/ban
set test actionunban iptables echo <ip> <cidr> >> /tmp/unban
get test actionban iptables
get test actionunban iptables
</pre>
Currently this records only the echo as the action.

> but that then also requires documentation (--help output) clarification..

So <pre>
sets the start command <CMD> of the action <ACT> for <JAIL>. Quote <CMD> with "" or '' if more than a one word command is required.
</pre>

> I do see it as a possible improvement,

>  but still wonder if it is worth it at all

make up your mind. Unless you knew about quoting you've got a barely usable set <jail> action* client interface.