Disadvantages of Bunk Beds
The biggest problem of bunk beds can be that it causes your bedroom to have a very crowded feel to it due to the large size of the bed. If you have a small room then the effect is multiplied, however if you have a large room then the size of the bunk bed will not really have much of an effect. In my experience sleeping on the top bunk, it can be very painful coming down the ladder in the morning when your muscles are all stiff and feel like they have zero energy output capability. You do get used to it after a couple of months but it is not very pleasant to have to climb down some vertical stairs first thing in the morning.
Another factor for consideration is the lack of sitting room it creates. The bottom bunk usually does not allow for any sitting up straight positions because the top bunk is too low and in the way. There are structures that come with the ability to turn the bottom mattress into a couch type seating area but it is an add on that does cost extra money. Sitting up on the top bunk is also inconvenient as there is not too much to do up there.
Things like watching television and playing computer games from up there can be a bit awkward. A single bed in contrast is very good seating area in almost any persons bedroom, especially for things like watching television and playing video games.