You can track your instruments!
That should sell you? no? 
Just like you can add envelopes and stuff now, using patterns you can do pretty much anything to your instrument. You can stack instrument/samples there (just use several columns/tracks in the instrument-pattern). Do multi fancy trackery tricky wizardry. Use pattern commands, add fx etc.
In other words, anything you can do to make a song in renoise now, you can also use to make an instrument.
You can read more about instrument-patterns and other cool suggestions in the pinned RNI Future thread.
The idea was to combine the instrument-pattern with traditional instrument/sample mapping. You just drag/drop the different elements to a key/velocity-mapping. So you are not forced to use the instrument pattern to assemble your instrument.
You can use plain samples (like now) or you can use entire instrument (choose to copy or to only link them to a new instrument), or you can use entire patterns. (and later do the same with clips).
It’s a very simple system really, where its up to you to choose at what level you want do assemble a instrument. I can see a lot of creative use of such a system.