This is actually more rational than what you are advocating.

Are you sure you understand what I'm advocating? Your claim here suggests to me that you do not.

The most probable outcome of the thought process I outlined, and I say most probable because this is what my reasoning concluded but Harry is smarter than me, is that he will go and earn enough enough money to buy a time turner just in case. I aren't advocating the use of the time turner to make reckless experiments or any of the non-authorized uses that Harry had previously used the device for. I actually made it quite clear what I thought of clever experimentation when explaining the note I would send myself. "Arrogant git" was the key phrase if I recall.

What kind of usage is sane? Emergency usage. For example, I would advocate the use of a time turner when locked in a room being tortured and at a high risk of suicide if I do not prevent it. (I liked the touch with the destroying all sharp objects too.) I would also advocate the use for saving lives (including his own depending on circumstances) with minimalist interventions, many of which would be not at all experimental given what he has already safely gotten away with. It would also be extremely valuable in the reduction of risk to have an extra 36 hours available for emergency use. Many threats to Harry's world optimisation plans will come with some warning. Impending attack of some sort, etc. Having a turner on hand means that he would be able to take more time to make preparations or give th device to McGonnagal in the emergency situation and allow her to prepare. As far as either myself or Harry know she isn't wise enough to have one on hand herself.

A time turner is a device that is massively useful and it is massively useful even if you use it conservatively and take no risks with it. Having one on hand does not make bad things happen unless you know you will be unable to control your foolish impulses. This is not Harry's reasoning.

If Harry does not spend at least five minutes thinking through his priorities (let me emphasise that again, spend five minutes) and considering how to acquire (ie. buy) a time turner then both he and the author are making a big mistake. That is *not more rational. It is a rationalization of a decision that was originally made for the purposes of plot balance.

The reasonable solution is clearly for Harry to do is to put "buy time turner" down on his to do list right next to "become ridiculously rich in a week". It is an obvious smart thing to do but the reader understands why Harry can't really follow through with these plans for story purposes. Having them judged "not a pressing priority" is a credible explanation.