What a terrible idea for me would be screwing up with people websites?
Right now the service is used by more than 5000 websites and gets just shy of 10m hits a month. If anything would happen quite a few people will know who is at fault. But when any popular internet service goes down you are just getting formal apology from nobody.
If they stop offering their service, just change your DNS record, and viola. I do not see how this could be a dangerous to you.
Shameless plug, but if you want instructions: http://bionicspirit.com/blog/2012/01/10/domains-merged.html
