Well of course it's "allowed" -- there's no Governing Board that's going to kick you out of the club if you spend on something that does not meet predetermined utility metrics.
Look, the point is to figure out the kind of life you want to live, understanding that we all tend to get swayed by consumer culture into thinking we want/need things that we really don't and that don't make us happy. Most of the time, we are absolute shit at identifying what will make us happy, and so we jump on the hedonic treadmill and chase more and bigger and better in search of the next dopamine hit. But sometimes, there is some ridiculously stupid thing that actually does make you happy. And if you've got one of those, then yes, you can and should pursue it -- just do so thoughtfully and methodically. That includes:
- Prioritize your needs over your wants -- and that includes Future You's needs. Live below your means. Make sure you are maxing out your tax-protected savings vehicles, and saving whatever else you need to to be able to retire when you'd like to. If you're not already doing that, you can't afford your expensive hobby.
- Put a lot of thought into why you like your expensive thing, and whether there are other ways to scratch that itch that are free or cheaper, or that bring in money. MMM wrote once about a friend who was a serious car guy who scratched the itch by buying older cars, rehabbing them, driving them for a while, and then selling them for a profit.
- If it comes down to this particular thing is IT, then do the math on how choosing that path affects your path to FIRE -- how much extra do you need in your 'stache to support that hobby, and how much longer do you need to work to add that amount to your 'stache? Is this particular thing really worth that much longer at your job? If so, then go for it. If not, then go back to step 1 and re-evaluate your budget.
- Keep in mind that often expensive hobbies travel in packs. Like, say, if you love golf and join a private club, after a while you may feel out of place driving your 15-yr-old Camry and wearing golf gear from Target, so first you start buying nicer clothes, then you suddenly need a nicer car, then you make new friends and start hanging out for drinks after rounds, and of course then your gear is insufficient, and the next thing you know, your golf habit is costing you multiples of what you originally budget. So if you do jump into an expensive world, you will need to work very, very hard and probably manage some internal discomfort to fight off hedonic adaptation.
- Remember that there is a big difference between "no" and "not now." You may not be able to afford your stupid spendy thing now. But things may change in a few years. If you put in the work now -- maxing savings, increasing your income, lowering your other expenses -- you may find yourself in a very different position in a few years. Hell, make it a personal goal, and create a separate savings account called "Flyingstache's Stupid Money" so that you can see you're making progress towards that goal.
FWIW, I have a StupidCar.* I continue to be amazed no one has booted me off here because of it. I've wanted it for more than 20 years, and we certainly had the income to "afford" payments on it. But I waited until we were FI and I could write a check without endangering either the kids' ability to go to college or DH's/my ability to walk away from work at the drop of a hat. And I have not regretted it for a single day -- I love love love driving, and there is just no other car like this, and it literally makes me smile every time I downshift into a corner (preferably with the top down). So yes, I get it. But I will also say that I enjoy the car so much because I waited and worked and saved for it. Because there is absolutely zero fear or guilt that that money should have gone to the house or the kids or some other family need; it's just pure enjoyment.
*Of course I bought it used. But it was still a massively stupid expense that is completely unjustifiable by any reasonable metric. I just don't care.