The pillar 1 are the pages: the websites.
The pillar 2 is being able to share big files reliably: BitTorrent.
The pillar 3 is direct communication: this is supposed to be e-mail.
Notice how these pillars turn I2P from just a network into a complete suite usable for all primary internet tasks. Including I2PSnark into the default Java client was a great idea because it made it possible for me to convince some randoms on the internet to use it to share some files they wanted instead of relying on crap like shitty random file hosts. There is definitely demand for anonymous decentralized mail. Being able to tell some internet rando to use I2P to communicate would also be very helpful.
The synergies between these pillars will make the whole I2P stronger. For example, if I want to add a large attachment to my I2P mail, I can do it as a torrent. In clearnet I would not do such a thing, because I don't want to reveal my IP, and NAT reachability is always a question. Furthermore I have no idea if the other side can torrent. But I2P is the enabler: If both e-mail and torrent is included in the default I2P suite, I can do all this without the aforementioned worries.
But who uses MuWire? Is this something that's actually used by people, maintained by the devs? Seems to be abandonware. This is why I insist on having this e-mail be accepted as a core pillar of I2P suite that works out of the box or not be done at all. I can maybe convince people to install I2P and click I2PSnark, but making them jump through the hoops to install a potential abandonware 3rd party thing? Fat chance. Also of course this e-mail thing needs to be as simple as possible for devs to be able to maintain it. No crazy complex features.