Personally, I would rather pay for a license and put it on my own server than pay 100\month and not own my data. When you are already paying for a server might as well use it. 
The nice thing about keeping a seperate email autoresponder provider is that it does not keep all eggs in one basket. Plus TW specializes in email marketing since the early 90s so one can imagine the level of the support and deliverability. To each their own, though. 
