Little bit of history...
Back in the day of skinny steel tubes... it was quite common, especially on really small bikes, to route cables this way, and the exposed derailleur cables crossed each other underneath the downtube. It made a much smoother arc in the housing from the handlebars to the downtube, which of course, results in less friction and better shifitng. And that was with 8 and 9 speed stuff. The only reason I didn't like it back then was an aesthetic one, I just really didn't like the look of cables crossing like that underneath the downtube... I much preferred the look of the exposed cables running along a straight line down their respective sides... "Mind the lanes, cables!". It just looked nicer to me, yet I understood the functional reasoning behind the crossing under the downtube method. I also liked to "frame" the headtube badge, be it a Colnago logo or whatever, between the elipse of the cables crossing in front of the headtube. Again, it was purely an aesthetic thing.
Then tubes started getting fatter, and derailleur housing bosses on the downtube were not low enough to allow cable crossings underneath the downtube, so that type of routing became impossible for awhile. Until the tubes got even fatter and internal cable routing became popular. All of a sudden, with the unsightly cable cross underneath the downtube now able to be hidden within the downtube itself, I really had no reason not to do it, and lot of reasons to do it...
1) It results in only one rather large radius bend from the shifter to the downtube... it's almost as if the housing is in a straight line. Bottom line is this reduces friction, a lot, compared to the tight radius S bends that have to occur otherwise. You can do a simple experiment to see the difference: take a new umounted shifter housing and insert a derailleur cable through it. Bend it into an S shape and pull the derailleur cable through. Now let it go and just make one large arc in it and do the same test. I don't think you even need to go through the exercise to realize which one is going to have less friction. Now, imagine squishing that S shape between the bars and the downtube of a small frame... ok... enough said.
2) If you buy the first point above, and you realize how adding more gears (11spds) have meant that each shift requires a smaller amount of cable pull than years gone by, and you also realize that the less friction the better, then from a functional perspective I can't see how anyone could argue against this routing.
3) From an aesthetic point of view, the crossing of cables occurs inside the downtube, so no more unsightly crossing underneatht the downtube. There is no added friction, unless you can convince me that a 1.2mm steel derailleur cable passing over another for 2-3mm, is being held up somehow. The only thing you have to be careful about is that the cables don't actually get twisted around each other inside the downtube, because that would indeed cause some shifting issues.
4) There is no point where the cables are rubbing on the headtube or other parts of the frame. Saying you can put a sleeve around the housing that rubs on the headtube has never sat well with me, because now instead of the actual cable housing rubbing the abrasive dirt into your headtube, you now have the sleeve doing that job. It does protect you cable housing however
. And I really hate those clear stick on patches where cables rub on the frame.
Ultimately, you can use whichever method you like... I'm currently using this method on virutally all my builds, except where it's impossible (My C50 for example, won't allow it).
Oh, and for the rear derailleur housing... that deserves just a brief mention. The key to the correct length for that will depend on your frame ports and bosses, as many rear derailleur cables just exit out fo the end of the chainstay now, versus being attached to a bosse several centimeters up and underneath the chainstay. Also, the SRAM derailleurs typically require a much larger loop so that the cable enters without too tight a radius. The new Shimnao derialleurs actually come with a much more flexible piece of derailleur housing just for this purpsoe, because the length to their derailleur entry point is so much shorter than it has been in the past.
Then, finally, from an aesthetic point of view... I strive to have all the cable housings intersect within a 5mm circle or so right below the center of the stem out front. If I see a cable job like that, I know the builder probably took the same meticulous care in putting together the rest of the bike. Sloppy cables, sloppy build.
