The reason why people should side with the reapers is because siding with them permanently changes the world where siding with golden sands just reverts it back to how it has been for the last few years (there will probably be slight differences in how it looks, but nothing compared to what will happen if the reapers win). Do you want to see the world stay the same or do you want to see the world change and evolve is the question you should really be asking when picking a side. A common argument is that Golden Sands is the best outpost, but Sanctuary is essentially the same. Many players want to have another outpost, but why have 1 more outpost when we could get to see what is done with it if reapers win and get something entirely new and interesting instead of old and boring?
Edit - For those who say that Golden Sands is just going to stay in ruin permanently, we are already seeing changes there depending on if the Reapers are winning where a phantoms appear and watch you. Also, you have to think about the residents of Golden Sands, they will need a home and will possibly make a new outpost (just speculation of a new outpost, no hard evidence) or something else will happen to them which will be interesting to see and find out.
