adding water during the boil

so i brewed my second batch last night. During the boil i decided to periodically add boiling water in order to keep the volume at a constant three gallons.

The rational was that as the wort reduces the gravity increases changing the utilization of the hops in the boil. Also i used all whole hops which soaked up quite a bit of the wort.

I've never really heard of this method being used, which leads me to believe there must be some downside to it, but it seems like maintaining a constant gravity during the boil should be a good thing.

I believe the only downside would be time spent. You will be diluting later anyway.
To optimize you AA conversion, you should look into Late Extract Addition