The number of DNS lookups for SPF records is limited to 10 [1] so that that could be a technical issue that needs to be worked around [2] depending on how many SPF records each service adds:
Sure you could do that, but I think the maintenance cost would by far outweigh any price advantage you could get.
Honestly, Mandrill's pricing with 0.2$/1000 mails puts you at 200 $ for sending 1 million e-mails, which corresponds to maybe 1-2 hours of developer time. Designing, building and maintaining a system that sends mail through 4-6 different APIs will likely be much more expensive.
No reason. It's probably a good idea to implement that even if you don't need the volume, so you can quickly switch if an offering changes or has issues (downtime, delivery problems for an important provider, ...)
[1] http://www.openspf.org/FAQ/Common_mistakes
[2] http://stackoverflow.com/questions/14261214/too-many-dns-loo...