This is always a tricky one.
We offer a 1 Off introductory offer for the duration of the period you sign up for.
Our prices are great value anyway, so we would hope that our existing customers stick around.
We've had the odd customer leave and then come back in the next 2 to 3 months, but we've never had any complaints that the introductory offers were unavailable to them.
I suppose the theory of giving offers to your existing customers to retain them seems plausible, but it becomes difficult to predict future cash flow.
I'm not sure that I would be too happy if all of my monthly paying customers only paid 50% for a given month because we opened the introductory offers to everyone.
If you are in the shared hosting business and your are charging around £10 for a shared account and £20 for a reseller account, then you don't have that bigger margin to play with. If you discount too heavily then how do you afford to provide the support to everyone?
Realistically, if we assume the server and power costs nothing. If a customer pays £20 a month and uses more than 1 hour of support, you as a host have lost money, as that engineer has cost more than the customer has paid.
I'd much rather that engineer spend his time ensuring that everything is working and that support is not necessary.