Thanks to open source, you can answer yourself: https://github.com/apache/cassandra/search?q=md5&unscoped_q=md5 At a glance, looks like it's used for digest verification, and to get a good hash distribution on the RandomPartitioner
I haven't done the math, but I suspect CRC32's just not good enough either in terms of result distribution for hashes or ability to catch multi-bit errors without accidental collision (that is, it doesn't sufficiently guarantee uniqueness). There are other error checking algorithms that are probably less computationally complex, but thanks to a lot of hardware and software optimizations geared towards md5 specifically over the years you'd be hard-pressed to find something that gives you comparable speed in practice for a 128-bit result.
