As for the comparison between pipelines and powerlines:
The biggest difference is in my eyes, that powerlines do not transport any "good". What they "transport" isn't essential to any production chain - it's just a bonus gadget to increase overall production. It is not necessary
to have for any
factory to work. And those powerlines are actually doing a job that no vehicle
All of that is supposed to different for pipelines. So, my (personal) conclusion: those two should not be compared as they are entirely different.
Ok now back to the issue itself.
I agree that pipelines would probably be both useful and realistic to have. But for me (and I think for many others as well) the fun in simutrans is not from having the factories working as a goal of itself - it's having your vehicles
transport everything to and from those factories and cities. I just don't see any of this fun in having a pipeline (as useful as they might be) just pumping things from A to B. (Compared to a nice vehicle network doing its dance, it is really rather boring IMO). After all we're talking about a game here - and games are all
about fun and enjoyment. And just because something is useful
in real life (and might also be useful
in game), it doesn't mean that it's really good
for the (fun of the) game.
However, the scenario that fabio came up with (about connecting only small distances, where vehicles wouldn't really fit in) has some merit. And I agree that, should pipelines come, they should be very
expensive to build (to discourage people from using them on long distances - for reasons of gameplay and fun, despite of maybe being unrealistic) and I think they should have a fairly high maintenance, too (allowing them only rather small profits, so that vehicle should stay the primary choice, pipelines only secondary).
On a more philosphical - and personal - note at the end: I imagine the player's company to be more like ... FedEx, DHL, Deutsche Bahn, JR Group, etc.... not Gazprom or Eon, etc. In other words: just a transport and logistics and/or trucking and/or railway company - not a mayor's office, not a production industry, no civil services, etc... I hope one can understand what I mean. And on that point, I (personally) would probably restrict pipelines (if they ever come) to be only possible to be built by the "joker" player - the public hand (And to stay in this philosophy, the same would of course be true for powerlines as well - here
they are actually the same