IMO, there's just no need for more than one power system unless the additional power system actually plays out differently. If it's nothing more than a multiplicative conversion, then you're not making your own power system, you're just renaming the units.
I'll try to give a brief summary of what's been happening in the BuildCraft land in regards to this.
The decision to move to RF came from a few things. For one, there was a large amount of issues with the attempted rewrite of MJ in BuildCraft 6; at the same time, BuildCraft was no longer in a position of being a "staple" mod to which other mods are willing to adapt. (I also won't deny a lot of the players on the server I initially ported BuildCraft to RF for wanted to play with Thermal Expansion alongside BC, but it wasn't a major factor in the decision.) Do keep in mind, also, that SpaceToad and CovertJaguar's opinion on how important MJ was to BuildCraft differ - the latter saw it as an important part of BC's balance, while the former added it in the first place mostly to replace the very laggy redstone pulses.
Now, the move to RF has meant some compromises for us. MJ was a two-dimensional system - it relied on pulses sent out periodically; therefore, both
strength and
time were usable values. (Admittedly, only the extraction pipes used
time in a meaningful way at that time in BuildCraft's development.) RF is
in practice one-dimensional; while there's nothing stopping you from using time as a factor yourself, most mods limit energy in terms of RF sent per tick as a form of balance, which makes this infeasible - it effectively creates RF which works very inefficiently with other RF machines, and that's not good for any side of the debate.
For a long time, I have been experimenting with ways to make RF in BuildCraft more interesting. I have added an option to restore power loss (even though I believe that distance-based power loss is a
terrible solution to the problem and that TE2's extraction-based, constant power loss was far better). I have also been evaluating simulating idle (or, to be more specific, per-connected-machine) power draw on top of the RF API to try and bring back the old concept of "perdition", as well as conveting from one RF form to another - sadly, none of these solutions proved fun or interesting and they either only added meaningless difficulty or were too confusing/unreliable in practice.
There still remains the issue of what other mods
do in regards to BuildCraft. For instance, one of the key balancing points of the "old guard trifecta", as I sometimes call it (BuildCraft, Forestry and Railcraft), has been to not allow long-term, lossless power storage in the form of batteries or energy cells. (Unfortunately, Railcraft has by now added an RF storage cart.) While I see nothing wrong in people adding a battery mod to BuildCraft myself (and, in fact, CovertJaguar, allaryin and I have been talking about various concepts for a short-term, lossy "capacitor" inside BuildCraft itself), there are probably people out there who are bothered by this.
Anyhow, as it stands, BuildCraft has chosen to create their own power system with some design changes as opposed to RF. One of them is that the internal unit is micro-MJs, or 1/100000th of an RF. This was requested by other modders who wanted to add things such as ultra-low-power (but not free) light sources which could be controlled by power and gates alone, without requiring an additional redstone signal due to energy buffering. Another change is that redstone engines are no longer a hack requiring you to implement a BuildCraft interface in addition to RF's own. Other things include passive (extracted from) and active (pushing into) power sources being differentiated in the source code and unifying particle effects for power consumption across the (hypothetical) BuildCraft ecosystem. As far as I am concerned, I had little say in the decision myself.
After I left BuildCraft, I designed some concepts of alternate power systems for Charset - sadly, it is unlikely they will see an implementation.
