I did the last 165 miles of the JMT last year with a Gossamer Gear Squall Classic (a spinnaker tarptent). Had no problems any night, however at Twin Ponds (just above Guitar Lake), I had to use rock as my stakes, but that was the only locale. I still used the stakes, the rock prevented the guyline ("Teed" by the stake) from slipping.
I'll be doing the entire JMT 4 weeks from now, using the same tent. Plenty of people hike the JMT using tarp tents with no problems nor complaints. Two of the other hikers had the large Tarptent 3 person tent, the Rainshadow 2 tent.
I hiked the High Sierra Trail for 7 years, of which 2 years where with the large Tarptent 3 person tent, the Rainshadow 2 tent. We used that at Twin Ponds too (just above guitar lake behind mt whitney). No problem (except, again, we used rocks to keep guylines down that night (otherwise stakes went into the ground just fine at all other camp sites).