Many of the healing issues, including this one, are difficult to really decide upon when monsters don't hit often enough.
As the playtest is right now, I think it is fine to eliminate in-combat healing. At low levels, average combats last about 2 rounds; many of them can be over in 1 round if the PCs are lucky (especially if they gain surprise or many of them win initiative). If the fighter and cleric are in positions that make it difficult for the opponents to attack the wizard and rogue, chances are that nobody will take any damage. At mid levels, pcs might get hit a little more, but they will have extra hit points so they should be able to survive 2 or 3 hits from any equal level monsters over a few rounds. As long as the DM doesn't swarm the PCs with overwhelming numbers, or attack with too many multi-attack monsters, or attack with a number of more powerful creatures, or purposefully attack the wizard or rogue, no healing will be necessary.
The problem is really the swinginess of any combat. When monsters don't hit too often, but when they do hit they do a lot of damage, that means that luck will play a huge role determining whether or not characters need healing. Everything could be great for 1, 2, 3, 4 or even 5 combats...but any single combat (with bad die rolling) could become a disaster leading to TPK.