I have the first series pioneers and 2 rw12d subs, along with some mirage nanosats for surrounds and front heights. all of my speakers cost about $350, the two subs cost $600, and it is far from overkill. if you think about it, for one, the subs come with an amp, the other speakers in your system do not have an amp, their power comes from the receiver. so a sub is basically to parts you are buying, an amp and a speaker. second, the low frequencies is what takes the most power to reproduce. treble is easy and takes less power, bass is harder with bigger waves and takes a lot more power. based on these two points, you need to spend more $ on reproducing the low frequencies vs the mid and higher frequencies.
i just added the second rw12d to my theater room about a week ago. the dispersion is much better. with only one sub before, if i stood in a certain spot, the bass was heavy, in another spot, the bass was weak. with 2 subs now, it is very even through out the room. all seating positions experience almost the same bass, where before some positions had too much bass and others were lacking bass. also, the feel is better. with 2 12" cones moving air, you can feel it more that with just one speaker compressing. a second sub did also add a little bit of volume, not a lot, but more importantly, it now has a flatter frequency response, goes a little lower, and is much more dispersed. i highly recommend to subs. good luck.