For me, a less than full power takeoff, when available space is not a concern, is a zero risk activity (you can always open the throttle to continue if you need to). The benifit for me of doing this in many aircraft types, is be more prepared for a partial power loss during a takeoff, when you are committed to make the best of it. One example of a 25% power loss during takeoff is a stuck exhaust valve on a four cylinder engine, which has happen to me three times. It also provides practice in handling finesse. I remind myself that the somewhat lighter cub I used to fly would get the same load airborne as my 150, and do it on 65 HP - do I need all of that mighty 100 HP? - It seems not! Where this is of the most value to me, is the real finesse required to get a floatplane or flying boat off the water. You can easily cook an engine dragging such an aircraft into the air after an excessive takeoff run. Much better to practice your technique partial power to hone you skills. It makes the availability of full power seem like JATO!
I could be guilty of complacency, if I did not do something to keep my skills up. Whenever I fly with an instructor by circumstance, I ask for a refresher, often I'm told "Oh, you're fine" after a circuit. A complement is nice, but not what I need, it's the challenge to my skills I need. So I gave up on getting it elsewhere, and do it for myself on a regular basis. The people whose aircraft I fly, and the companies who insure those aircraft expect me to keep my skills sharp, so I do.
I think that practice and training are only futile when they are unsafe, wrong, and encouraging poor technique, other than that, if it's safe, and you learn, go for it!
Pilot DAR