True, but when you have one house your also keen to have the gap between value and what you owe grow quickly as a lower LTV means lower rates on your mortgage which means lower payments and more ability to save and/or overpay, which increases your ability to keep up with the growing price of future house moves.
There is a massive difference between >90% LTV and <80% LTV.
Obviously if your just stick to the same mortgage and don't move about then you are in the position you describe. But if your on top of it and moving around to get the best rate and overpaying as much as possible, you can make the most of price rises on a single house. After all, its not worth having money in a savings account at the moment, so the best place for it is in your property.