I'm not sure whether it would be better to be sealed to keep humid air out (that might condense in the pipe), or to have it open to allow airflow to allow moisture to evaporate.
My opinion though is to make sure the conduit can drain at it's low point. The pipe I had trouble with was pretty straight and level under-ground, but at the dish, and where it entered the house, I had elbows that brought it up to ground level. The soil there was fairly dry most of the time, but I think water followed the wires down into the pipe at the dish, and possibly also at the house, even though I think I had a rock covering the pipe at the house. But when I upgraded my dish, I went to replace the coax, and the whole pipe was pretty much full of water, and the ribbon cable was all slimy. The cable was supposed to be direct burial stuff, and normally you wouldn't worry about water getting through the waterproof insulation, but I don't think that necessarily applies to being under water 100% of the time, never getting a chance to dry out. That cable I pulled out felt slimy and looked degraded even after it was dried out.
My current conduit, at a different house, has a significant slope, and ends in a hole by my garage, that drains pretty well, except in winter, so it stays pretty dry. If it were level, I think I would have put in that pipe with holes in it, so that the water could drain out. But basically, if you can remove sharp rocks, I think the only reason for conduit is to make it easier to add new wires without digging again. But if you did what the other poster did, and put in additional wires for future use, that maybe it might be better to just bury the cable without conduit. I'm glad I put conduit in though, because I've added wires on about 3 occasions, plus I didn't remove all the sharp rocks, and I drive tractors and other vehicles over the wires all the time, and the conduit protects them from physical damage.