I feel like this is not a problem right now though. Back when the rule was introduced, what you're describing was a common occurrence, but I think most players today either put effort into their house or... don't, but whether the house if a spawnhouse or not won't change that. I also feel like spawnhouses are deeply ingrained in the concept of Lava Survival-- As far as I know, spawnhouses were always allowed during the time CLava was up. Considering the server is, in some ways, going back to its roots, I think it's a good reason to allow them again. Maybe if what you're describing becomes an actual problem again, disallow them, but until then, why not have them be legal again (certainly on Classic rounds- which are the nearest replica to CLava we'll ever get)
EDIT: Another spawnhouse-related issue: A lot of map spawns are broken in a way that makes people unable to create an actual spawnhouse after it's been flooded. All the blocks around the spawn melt, making it impossible for someone to actually build anything at all.