It's a personal choice, not worth the headache of fiddling with the stock tim, the cheap plastic pushpins plagued with problems, the potential for loud noise when the fan ramps up under load. Me personally, that stock chunk of metal never even makes it out of the box, it's not worth my time. A 6500 does come with a stock cooler so you could save $30 or so there. Granted with the k series you 'have' to buy a cooler since it doesn't come with one. The 6600k itself is only around $20 more than an i5 6500. It also depends on which cooler, a $35 cryorig h7 or $100 h100i? In other words it 'can' be a lot more expensive to overclock but it doesn't 'have' to be based on parts chosen. I can make it a $500 difference if I go with something redonkulous like a $500 z board like the gigabyte gaming g1. So is it a $150 investment over the top of a non k? Depends what parts you choose really. There are cheap z170 boards for $90-100 and solid oc choices in the form of $120 z170 boards. A $160 motherboard isn't a 'requirement' to overclock. Not to mention the performance that people were waiting for to release with skylake I was enjoying for a good 12-18mo before it even hit store shelves. Not counting having to buy ddr4 ram, having to reinstall windows etc. The fewer times I have to do that, the better. It would mean a $200 cpu and another $130-150 motherboard. On the other hand, If I had to upgrade to skylake it would mean more than $30-50 for a decent cooler and $20-30 for a nicer motherboard. By going with a big air cooler even oc'd my i5 is whisper quiet and I like that.

Not everyone cares what it looks like or cares about noise.

I planned already to use an aftermarket cooler, for looks and for performance. It meant a z series board and a better cooler which were a bit more. With my 4690k overclocked I can easily get i5 6600 performance from it. A cpu that's 'enough' tends to struggle sooner or later and by the time that happens a generation or so of cpu's and their motherboard sockets have passed meaning a new replacement is rather costly.

Benefits of overclocking cpu