Looking on the internet deeply has found these results:

I have interest. is the most popular phrase on the web.

I do have interest. 185,100 results on the web	I have interest. 1361,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I'm allowed to have interests. Unlike some of our founding families I don't have conflicting interests. Yes, Lennox Gardens is my house, so I do have a vested interest. Maybe I don't want to meet Someone who ShareS my intereStS. I hate my intereStS. An old lighthouse like Nora's is bound to have interest. Of course, I don't yet have controlling interest. Otherwise we shall find we are short of majorities on a number of votes where people legitimately have interests. I don't have conflicting interests.	You and I have common interests. What's changed is I have completely lost interest. I could devote the time I have left to other interests. I'm allowed to have interests. So, yes, I have taken an interest. It is message sent to you, which I have interest in. Yet I have many vested interests. It seems you and I have mutual interests.

I do have interest.

185,100 results on the web

I have interest.

1361,000 results on the web

More popular!

I'm allowed to have interests.
Unlike some of our founding families I don't have conflicting interests.
Yes, Lennox Gardens is my house, so I do have a vested interest.
Maybe I don't want to meet Someone who ShareS my intereStS. I hate my intereStS.
An old lighthouse like Nora's is bound to have interest.
Of course, I don't yet have controlling interest.
Otherwise we shall find we are short of majorities on a number of votes where people legitimately have interests.
I don't have conflicting interests.