Question about MySQL Semisynchronous Replication
Baron Schwartz wrote an excellent article on this, and I think one key sentence answers your question:


  What does semi-synchronous replication guarantee me then? If there are no errors or timeouts, then the guarantee is essentially that only one transaction per client is likely to be lost if the master crashes.


So semi-synchronous replication minimises (but doesn't entirely eliminate) the chance of this exact problem:


Replicant B's replication IO Thread starts running behind its master - Server A - for some reason
Server A dies - unrecoverably
Transactions are lost forever


As noted in Baron's post (and based on your wording I'm sure you're aware of this), it isn't bulletproof - MySQL will fall back to regular replication if it stops getting a response from the replicants - but irrecoverable data loss is less likely to occur.

The upshot is that it prevents the situation above, it's "out of the box", and it doesn't require any third-party or infrastructure-level solutions (e.g. Galera/Xtradb Cluster, or DRBD). Whether the benefits are worth the cost is likely to vary depending on your situation.