This 'seal v let water through' argument has been discussed many times.
There are many comments saying you shouldn't "because blah", and many that say it's better to seal "because blah..."
I have seen some posted photo's of 'sealed', and some that haven't.
What I have never seen...... is someone reporting they have actually had a problem caused by sealing, which suggests to me that it isn't actually a problem.
however what I have seen is someone on here who used the BMW treatment only to find green mould growing later on.