Yes, a motivated attacker could figure it out. But before they could do so, they would need my password in plaintext for multiple unrelated accounts of mine, which is hard, requires the attacker to target me specifically, and by that point what is lastpass really going to do for me anyway?

By far the most likely way my gmail account would be hacked is that foo.com's database is leaked/cracked, and the hackers spam the credentials for foo.com at hundreds of other sites and see what sticks. My scheme defeats that. And it's one point of failure versus several.