MikeFerrara:The problem with sending a diver up alone is that the descent and the ascent are two of the most likely times during a dive to have a problem as is any other dynamic portion of the dive.
No one should "cut your dive short". You plan how the dive will be conducted which includes setting time and gas limits and you dive it that way. If the DM has a buddy he should stay with the buddy. Otherwise he isn't a buddy is he? If you aren't able to dive without a DM then you should go up too.
When I go somewhere with a buddy to dive, it is understood that both of us end the dive when either has a problem or is at the turn pressure. I have had the mentioned problem though when I go on vacation and go to dive "alone" (alone meaning my buddy is not diving that day and I am going to be assigned a buddy when I arrive at the boat, no I am not diving solo, though I have had assigned buddy dives turn into "solo" dives with a group). But, I think that running into a situation where I have to surface on my own is part of the risk I take when not going with a buddy to dive. I am not going to rely on my assigned buddy for jack, though I obviously will stick with them and assist if needed, and will surface with them. I think it is dangerous if you expect much in return of someone you don't know.