A READ ONLY option would allow you to preserve your SSD drive while still getting a useful test rather than ALWAYS having to run a WRITE test. I would prefer to run the WRITE tests only once in a while, using the READ test to get a quick idea of how the SSD is performing without putting wear on the drive.
And, some users put READ only files on their SSD, so the WRITE test is not needed in those cases.
So, in general, a READ only option in preferences would be useful for a number of reasons.
hiyohiyo 
CrystalDiskMark
A READ ONLY option would allow you to preserve your SSD drive while still getting a useful test rather than ALWAYS having to run a WRITE test. I would prefer to run the WRITE tests only once in a while, using the READ test to get a quick idea of how the SSD is performing without putting wear on the drive.
And, some users put READ only files on their SSD, so the WRITE test is not needed in those cases.
So, in general, a READ only option in preferences would be useful for a number of reasons.