As defender and promoter of the Low End, I realize this thread has gone off course (but not necessarily in a bad way

).
After further review on the subject of Flats and Rounds, I've found a few more interesting comments to add...
While I agree that rounds are suitable to the task of keeping the low end tight, there are some things about them that, to me, are still inferior to Flats with regards to a tight rhythm section (in Rock music specifically).
Rounds sustain better than Flats - This is very important with regards to a tight rhythm section. Tight rhythm sections require synchronicity between the bass and drums. Sustain is not viewed as a quality here. How long a note rings out can be controlled with proper technique so this should not be an issue, yet there is still some merit to my point favoring Flats.
Flats' sound decays faster than Rounds - This, to me, is one of the single greatest strengths of Flats in a tight rhythm section. This decay contributes to their more 'percussive' quality. This is one area where Flats greatly outdo Rounds. Because each note cuts out quicker, it matches the drums natural decay better, even without having to use technique to control it.
Rounds have more harmonics and overtones/noise - This is one area where many prefer using roundwounds. It is also where Rounds excel at giving 'bass players' more freedom of expression in the area of the guitars. Combined with more sustain, it is easier to obtain that growly sound as the windings produce more 'bite', and with less fundamental but more harmonics, 'bass players' can do more chordal work and more soloing, use more effects (to a greater affect) and it opens the possibilities for more variety in tonal quality, without things getting too muddy. Not a bad thing for those who like wanking and playing chords. But not a good thing for a very tight rhythm section, unless a lot of control is used along with great technique.
Flats have a stronger fundamental with less harmonics, overtones and no noise - Flats produce a nice, fat, deep 'thud' or 'thump' that is so interconnected with the drums. Each note is felt more strongly than a roundwound string at the same volume. With less harmonics, there is less extraneous sound produced to muddy up the low end or compete with guitars. Flats strength in this area is to produce a strong note that ties in with the drums.
Rounds have a piano like ring and clarity - There is no question that Rounds give a sheen to the notes that is very melodic. But again, I am talking about strings and how they relate to a tight rhythm section, so why would one use a melodic sounding string to help tighten the rhythm section?
Flats have a distinct percussive clarity, more akin to a xylophone than the piano - Probably the main reason I prefer Flats to Rounds for a tight rhythm section in Rock music is this quality. Every note produced by a bass using Flats has a distinct attack that is so much more closely related to the drums in a stringed instrument. Rather than having that melodic quality of a roundwound, Flats produce a more rhythmic, percussive sound that keeps it tightly within the rhythm section. This is what allows other instruments to play over the top clearly without a competition between guitars and bass for sonic space.
I do believe that Flats are better for a tighter rhythm section than Rounds. They simply fit better with what the drums are doing and keep the bass staying back in the low end of the rhythm section, preventing potential competition with guitars and other mid-ranged instruments.
Now,of course if one wants to play more chords, more open melodic bass lines, solos...or just wants their bass to be more of a low end guitar then there is no doubt Rounds will excel better than Flats for that.
Both types of strings serve their purpose. If one looks back at the history of bass strings, roundwounds came about due to the demand of bass players wanting to do more with their instrument than just playing 'bass'. Entwistle comes to mind with his desire to play the bass 'guitar' as a low end guitar (his words, not mine). This also came at a time where the technology of bass amplification was nowhere near what it is now. Bass players wanted to be heard and with rounds' clarity, and sustain those strings fulfilled a specific purpose, and still do to this day.
But, with the advancement of bass amplification, onboard preamps, active basses, etc. as well as string making technology, Flatwounds no longer need to sound like some worn out rubber band on a washtub bass. Flats today just give a rock band's rhythm section more balls, more thunder and yet still are bright enough to give each low end note very distinct attack and definition. Played with a pick, that attack is even stronger.
Of course, this isn't to say one can't have a tight rhythm section playing with roundwounds...far from it. I've spent the better part of 30 years doing just that. However, Flats will make for a stronger, tighter, more powerful rhythm section...all else being equal. It's just a matter of where one's priorities lie.
Just my opinion.