One thing that I didn't see mentioned here, which I think that a 3-way bookshelf speaker could improve upon over say the Sierra 2 is an even wider off axis response that closely matches the on axis response. Don't get me wrong, I love my Sierra 2s and I don't think I would purchase a 3-way bookshelf version of that speaker just to improve the off axis performance. The Sierra 2 off axis response is very good. But, it is striking though when you compare the Sierra tower or Horizon with the Raal ribbon tweeter with the Sierra 2, in terms of how much the dedicated midrange helps in getting a very wide off axis response that is nearly identical to the on axis response, especially in the critical midrange.
Now that the Sierra Luna LRC is getting close to production, I think that it will address this issue though. Looking at the off axis measurements of the Luna vs. the Sierra 2, we can see the improvement I was discussing in the Luna because of the smaller woofer and the narrower front baffle. However, the Luna LCR will have two woofers to get the efficiency back to the level of the Sierra 2s. The only thing that you seem to lose out on with the two smaller woofers compared to one large woofer is bass response, likely because the Sierra 2 cabinet volume is about double that of the Luna LCR.
Maybe those looking for a three way Sierra bookshelf speaker could get what they are after just by going with the Sierra LCR coming soon. I'm not implying that the Sierra Luna LCR is a 3-way (or 2.5-way) design, but that you will likely get at least the one mentioned benefit of a dedicated midrange because of the smaller woofer and front baffle.