If people just move their email hosting, then DH has less incentive to provide a good service. Customers should demand a good email service, live with a bad email service, or move completely to a different provider.
For 99% of people on DH (hobbyists, personal web sites, or very small businesses) paying for two services isn't a real option.
Arguably for bigger businesses the cost of paying for two services is negligible -- but in principle, it would be nice to expect a good service.
And I think overall DH is very good - have been with them about 9 years -- but they're poor in 4 areas: webmail offering, email reliability, old/unsupported 1-click installs, initial customer service if account creation isn't automated.