The rationale seems to be that shared IPs are cheaper for them (and the customer). It's also possible that Postmark doesn't have the capacity to operate a large number of dedicated IPs for individual customers.

Any large-scale sender for whom deliverability is critical should be using a dedicated IP. By virtue of hosting multiple customers, shared IPs appear to send more frequently, and any behavior on a shared IP resulting in a blacklist entry affects everyone else on that IP.

We're pretty aware of this problem at SendGrid. Even though we make every effort to cull bad senders from our shared pools, our enterprise senders nearly always prefer dedicated IPs, which let them build up a trusted reputation without interference from others.