I think the idea is definitely viable. I've mostly moved beyond BuildCraft (and RedPower) in my most recent 152ngt world and have the following observations:
1. Item management is handled far better and more precisely using other mods. Even if you were an Applied Energistics purist who didn't want to take advantage of all of the other tools available, you can set up perfectly viable inventory management using a secondary AE network with no interface and only a storage bus on the machine you want to manage with level emitters. Of course, packs that have access to Logistics Pipes makes this even easier. You lose the ability to evaluate several states of inventories for True/False, but my experience was that True/False was cumbersome and always involved building (what is now unnecessary) a backup loop to keep things from falling everywhere. All of that being said, BuildCraft pipes have definite value for some setups. I just haven't needed them to accomplish anything that I haven't been able to think of an alternative for in a while. Well... except for maybe a "Status Board" for a command center that turns a light on or off depending on if the machine is doing work or not. Far and away, the two most important blocks in the BuildCraft catalogue of item management for me are the Void Transport Pipe and Waterproof Void Transport Pipe.
2. Power generation is completely outclassed by other mods. Of the three stock engines, the only one I would really give serious consideration to building is the Redstone Engine -- and that is to power SUPER early game things like a Chest>Wooden Pipe>Void Pipe item destroyer.... which would not be possible in the first place without BuildCraft. The Thermal Expansion steam engine was buffed in the patch that made Conduit and Cells loss-less and it's hard to say it's not the kind of early game MJ production now. Even the Hobbiest Steam Engine doesn't really hold a candle to it anymore. For those people who happen to play with Universal Electricity, the options in Mekanism are also attractive.
3. I was hesitant about the loss of the Pump briefly, but then I considered what I've used it for recently. With Liquiducts and their forced extraction mode, the only liquids I've needed to use a pump for are Lava and Oil. There is now a smoother option for Lava, and Oil is irrelevant without BuildCraft.
4. The loss of the Quarry will hit everyone differently. Does the advanced quarry mod function without BuildCraft? I haven't looked too much into it. Even failing that, people who use IC2 have access to a more cumbersome auto-miner. I'll admit that this is probably the most prominent element of BuildCraft in 152ngt for me. Even then, I'm hoping to eliminate it entirely in my next world -- relying on Mekanism's ore tripling abilities and manual mining to get me to a power state in which I can commission an MFR Mining Laser -- those things are just awesome! Of course, we'd lose the best way to get the laser a clear sight to Bedrock -- the BuildCraft Mining Well.
All things said -- I think it's definitely something that could be done without too many headaches. Of course, the BuildCraft team aren't exactly ignoring the times. Losing BuildCraft also loses the potentially awesome things that might come out of it in the future.... although that doesn't mean you can't install it when you feel like it.
