You can rent the lens as needed anytime for $190 for 3 days from LensRentals.com. Anything else for a hobby photographer is pure waste.
I have to disagree with this, especially as you make it a blanket statement for hobby photographers in general. I can agree with it so long as the hobbyist only wants to use the lens maybe once or twice a year, but how many of us who enjoy this hobby only want to shoot every once in a blue moon. Unless the lens is very specialist, and wanted for just a single shoot, I don't see renting as the cheap option. The 300 f/2.8 is a very versatile lens and has many uses, I use mine almost every week for a wide range of subjects. If I was renting I would have the option of renting it almost every week (at a cost of maybe $8,000 a year) to use it as I want, or just a few times a year (maybe $1,000 a year) and miss getting a lot of the shots I get. I would also miss out on a lot of the pleasure I get from using that lens, and have the hassle of ordering it and sending it back, hoping the weather is what I want when I actually have it in my hands. As it is, I can decide on a moments notice that the weather, and the light, is good and just grab my gear and go out to shoot.
So, how much money have I "wasted" by buying the lens instead of renting? Well, I bought it brand new about 8 years ago for £2,995 and now, thanks to the new price rising over the years, I could get around £2,800 for it as a used lens. That is just £200 less than it cost me, or about the cost of ONE rental. So for the cost of one or two 3 day rentals I have had the use of the lens for 8 years, and hopefully a few more years to come. OK, sure, I have got the £3k tied up in the lens until I sell it, and that could be getting me a few quid a year in interest but I would be getting far less pleasure from it that way. If I sold the lens tomorrow, I would have had 8 years use of it instead of just a few days for the same cost.
My 100-400L cost me £895, new, 10 years ago and I sold it towards the end of last year for £775. A loss of £120 or just £12 a year.
To get back to the OP's question. I agree with all those who say that if you can afford it, then spending money on a hobby needs no other justification than the pleasure you get from it. I know people who spend many thousands on their hobbies every year, often in ways that once the money is spent it is gone for good (such as golfers paying big money for every round they play). At least with lenses the value holds up well in the long term.
I would have no problem justifying buying an expensive lens that will give me a great deal of pleasure over the time I own it, and depreciate (in the long term) just a few pounds a year.