What I think Thom is getting at is that if you're going to champion Cacti integration, you'll have to extend Cacti a bit to send DCE events, so that DataLogger can record them, and the rest of LMCE can act on them. So, instead of Cacti sending an e-mail or SMS when a threshold is exceeded, it would use MessageSend to emit an LMCE event which Datalogger would record, and that the system could act on if a scenario was defined. Like so...
Power usage goes over 5kw, Cacti generates a MessageSend to LMCE, Datalogger records the threshold exceeded event, and an "excessive power use" scenario is triggered, which broadcasts a message to all orbitors, and a "chi-Ching" sound is played over all audio devices.
So, it means some work extending Cacti, some work extending Datalogger, and some work figuring out events. Plus, doing all that in such a way that someone else can extend your work to graph other things (soil moisture readings, humidity, Squid proxy results for domains, etc). It would also probably involve integrating Cacti's web GUI into the LMCE web GUI, and use LMCE defined users.
I agree with you that Cacti and rrd's are great for recording some types of data, like multiple temperature sensors. Assuming OWFS as a sensing infrastructure, it samples by default every 10 seconds per sensor. So, that works out to 8640 samples per sensor per day. Over time, those samples can be averaged because the further away they are from the present, the less need there is for precision (which is what RRD's do). Cacti is also great at dealing with all the noise that syslog can generate. Same with other types of repetitively sampled data, where the need for precision becomes less important over time.
Where I think Datalogger integration could really shine is for correlating events from multiple subsystems into a "timeline of events". It's the concept of federated data; one overseer of other more specialized reporting/monitoring systems. It could be useful for debugging complex scenerios (this motion event triggered that lighting event when this other condition existed) as well as for security auditing (think alarm system event log), telephony logging, MythTV events, etc. It would be especially nice if the Datalogger events could be viewed from an orbitor. What would be really cool is heuristics to mine the Datalogger for patterns. Think a "vacation mode" which operates lights based on the inhabitants past behaviours.
Just my $0.02 CDN as someone who's presently dealing with Federating configuration and compliance management systems at work (in addition to other things).