I think the problem of where to put them stems from having applications as an MVC 'stack', as jrockway said in a sibling thread. Assuming that there is only one input/output point in a web-app is not very realistic. It seems that these applications might fit better in a hexagonal architecture[1], but Rails doesn't really support apps like that, so you have to cram mailers in where you can.
I can attest to that. We walked down the "One MVC" stack idea which simply doesn't work if an application has various modalities like Email, SMS, and Twitter. It turns out Rails view/controller stack is awesome for web applications, but only web applications.
For different modalities, like Email, SMS, or Twitter interactions, do NOT attempt to hack it into rails; rather, reuse the models defined in a Rails project and implement or use a different view/controller stack better suited for those environments.
[1] http://c2.com/cgi/wiki?HexagonalArchitecture