What would be a legitimate reason to have more than one email platform?
- Cost: prices increased with the number of email contacts in your database. So to keep costs down, many would split their lists and join another ESP.
- Multiple mail streams: having different ESPs will give you different IP addresses. Now, this is important because if you send transaction emails on one IP address, and those emails are opened and interacted with, your reputation will remain high (not stuck in spam filters etc). However, if you bulk send your newsletter, and the engagement rate is much lower, the reputation would be low on that IP address (more likely to be stuck in spam filters).
For each additional ESP, database coordination increases exponentially.