Marydoll
Our lawyer advised against it, when we were updating our wills.
So did ours. If memory serves, it was because if one spouse is widowed, they are left with half of the couple's money, whereas a joint tenant is left with all of it. Depending on the age at which the surviving spouse is widowed, a cut of 50% could have serious repercussions for life choices.
It might mean that half of the estate is protected from being used to pay for the care of the first to need it, but if care is also needed for the surviving spouse s/he only has 50% of their collective money to pay for it, so is likely to end up in a home that is not as good as the one they could have had.
It may be that the children (or other heirs) who have inherited the 50% will use the money to make up the difference and pay towards the home, but an agreement that this will happen means that they have to hang on to it untouched until the second parent dies. This could have tax and other financial implications for them, particularly in the event of a divorce, as the money would be seen as a marital asset. It is also unenforceable, as it would be the epitome of deprivation of assets.
Overall, given the absence of a reliable crystal ball, it is safer to keep the estate as jointly owned, as far more people do not need care as need it, and being left with 50% of your own assets is only likely not to matter if there is a lot of money in the pot. Most 'ordinary people' would feel the pinch, particularly when on a fixed income such as a pension.