One thing that would be a big issue for me personally, although I'm not seeing anyone else mention it, is the issue of migrating a world from 1.4.7 packs to 1.5.1.
TLDR: I don't want to lose RP2 not because I'm married to that mod, but because I want the modpack to be stable enough to let me keep the same world. Also, I don't see any replacements for frames.
I'm on a small private server with a group of friends. We used to just make our own modpacks by spending a lot of time and energy to go out, download tons of mods (our last pack was about the same size as FTB Ultimate), and manually check each one for compatibility, and then have to manually adjust the configs / ids / etc. so they would work together. Since we don't know java and didn't want to spend hours researching each mod, we couldn't fully optimize the packs and often had issues with memory leaks, and other problems.
The biggest issue is that, while we all want to develop the mods to their fullest, we never got to the high tech stuff (like quantum suits, nuclear power, etc.). The cycle went like this: 1) we'd make a new pack, and play happily for a few weeks. 2) We'd get busy with other issues, play other games, and end up taking a break from MC for a few months or more. 3) We'd come back to MC, and while we still had the old pack / server, we'd see all the cool new mods, features, etc. that we're missing out on, and figure "well, since we've been out of the game anyway, let's just restart."
Finally we found FTB, and when the Ultimate pack came out (which had most of the mods we wanted), and after a few tweaks, we switched (and restarted again). We gave up being able to have a fully customized modpack in exchange for the idea that a 'professionally' made one like FTB would be better optimized, and more importantly, would be designed to update.
So if FTB updates without RP2, here are our options:
- Just don't update. Miss out on any new features or new mods available, get alienated from the rest of the MC community ("what do you mean you don't know what a [insert 1.6. item here] is?"). Imagine still being stuck on 1.2.5, like users of a certain other mod pack. Also if you're trying to learn a new mod, good luck trying to find old info because now, you can't just google "how to use applied energistics," you have to go find an outdated video or article for the specific version you have. Also if any other friends want to join later on, we'd have to find a way to send them our specific version of the modpack to manually install (assuming FTB doesn't have a legacy pack install option).
- Start over AGAIN. I can't tell you how frustrating this is. I've never made a quantum suit. Yes, I said it. I have to keep restarting before I get to that point.* I see youtube videos of people with amazing high-tech bases running multiple nuclear reactors with a top-tier auto-sorting system fed by huge RP2 frame miners. I don't have huge amounts of time to play games, and I'm not going to just play MC for months on end, so this is a place I must get to over a long time.
- Try to port the world over anyway. With most other mods, this wouldn't be a big deal. I could replace macerators with pulverizors or vice versa. I could pull any assets out of other mystcraft dimensions and just port the overworld. I could use MCedit to put my base in a different biome if ExtraBiomes was going away. But RP2 is pervasive. It has invaded my whole world. I would have to find a way to replace all the RP2 World blocks (volcanoes, marble, etc.) and find a suitable alternative. We'd have to rebuild any tube-based machines we have (and we use tubes more than buildcraft pipes these days). I'd have to build a whole new mining system (I have frame miners), in addition to a new ore-processing system. Any buildings made with RP2 microblocks need to be rebuilt or fixed up. Sure I could use MC edit and creative to make this process easier, but then that taints the "legit-ness" of the world, especially if the other mod's system is more 'expensive.' This is just sounding like option 2 again.
On top of that, if we do upgrade, I don't know of any other mod with a system like RP2 frames. If there is one, please let me know and give me a link.
Sure there's replacements for computers, it's not hard for any mod to add marble and a dark stone like basalt, other mods are starting to come up with nice wiring systems, there's other options for microblocks, and blutricity is totally unnecessary when we have other power options. But what about all of the neat things you can do with frames, that no other mod (as far as I know) can?
Complain all you want about RP2 being closed source, it not being updated enough, etc. but the fact is, it's so pervasive and so loved because of the functionality it adds. I personally think FTB should hold up until it's ready. If Elo abandons it, then sure we will have to move on (although I'd hope in that case she'd open it up for other modders to continue), but I think it's worth waiting for an extra few weeks, even a few months.
Forking the packs would be another option of course, although it would involve some complications (will they be re-merged? What about the RP2 replacement mods if they are? Are we really going to have "Ultimate, Ultimate without Redpower, Direwolf, Direwolf without Redpower, etc.?") Then there's the slippery slope issue where people will ask for this treatment with other mods, and that could quickly become a mess.
*Full disclosure: I go for the Modular Powersuits armor first, and I have made that with nearly all upgrades 
