I play with 8 characters and deliberately went for a single Battle.net account, for many of the reasons you've mentioned: having a single mount collection, pet collection, heirloom collection (and soon a wardrobe as well), account-wide items (e.g. pet charms, certain reputation tokens) and achievements (e.g. Draenor Pathfinder) to be available for all of my characters.

I did find the prospect of playing for free with the WoW Token rather appealing as the cost of game time does indeed add up, but when it comes down to it, as elaborate a hobby it might be, I still want it to be just that: a hobby. I don't want to spend my time farming things multiple times for each of my Battle.net accounts. And while I'm doing quite alright gold-wise and often enjoy the impulses and antics of my inner goblin, I want those to be optional activities as well. There are some people I play with who frequently go "I have to go do x because I'll need to have earned amount y in gold again by the end of the month", and that's not something I want to have to deal with. Over a decade ago I was a rather hardcore EVE Online player, and while I have some fond memories of those years, I don't ever want to go back to a point where a game ends up feeling like a job.

There are some other things I do to help me cut costs though, as there's no point in wasting money either. I buy my game time cards in bulk, which makes the cost comparable to 6-month subscriptions, but allows for more flexibility on my part: as the codes are 60-day time codes, I can shut (some) accounts down without locking myself into 6-month spurts. To facilitate this, I've also set things up in such a way that I can temporarily scale back to less accounts without feeling like I'm severely gimping myself. For example, all my guild banks are all located on the first two accounts, along with the characters I'd consider 'mains'. I do buy WoW Tokens of course, and will just keep my fingers crossed that the restrictions will be lifted at some point, or at least loosened up.

In any case, that's why I personally prefer a single Battle.net account. I hope this gives you some food for thought to decide what works best for you.