What would you suggest as high end consumers that'd justify building a robust power network? On the MJ side, there's extra bees machines and the MFR laser drill. On the EU side, there's teleporters, mass fabricator(matter fabricator & interdimensional teleporters w/ GT) and again the MFR laser drill. I like uu matter for the simple reason that it provides access to blocks that are otherwise unobtainable (still water for example), whereas the laser drill just produces ores.
I'd love to see some more high end consumers that are purely for fun. Maybe a fireworks generator. Decorative lasers. Decorative block fabricators. Build pattern assemblers. The problem as I see it is that an energy consuming block has to do something some other block doesn't already do cheaper. I see some people play FTB and get bored once they've built all the machines. It's possible these people simply don't understand the basic point of minecraft (mine, craft/build). But giving them a fun target to keep playing with after they've built a power network could help.
But thats kind of my point: You are not "playing with" a MassFab. It only gives you raw materials. If you didn't have the creativity to do something with your raw materials before, giving you additional materials (like still water) isn't going to change that. You find the Massfab usefull because it allows you to be more creative, but thats not the massfab giving you new gameplay.
Teleportation is one interesting mechanic, though Mystcraft can do that, and very cheap. Forcefields and the corresponding defense stations are another fun application for multiplayer (but horribly OP and not very well implemented atm). Some other ideas: Really fast electric carts, Lasers are actually a really good idea, How about a device for large-scale terraforming? Basically it comes down to Multiplayer/PVP orientation or reshaping your world. Something as powerfull as WorldEdit, but with a power requirement might actually be cool.