There seem to be two very general schools of thought on pony tack, basically breaking down into the "as much tack as you can fit on the pony without him collapsing" side and those who prefer a more minimalist approach (probably most ponies and trainers fall somewhere in the middle).
Neither is right nor wrong (actually they're both technically right in that it is right for them), but what really goes into deciding how much (or how little) tack to use?
I'll start off with my own personal preferences. To be completely honest, I'm obsessed with pony gear. All of it: the bits, harnesses, masks, hooves, bridles, tails...everything.
Part of this comes down to mindset I suspect (the other part probably comes from owning bio-horses, where you can never have too much tack). See, what draws me to pony play is the combination of being dehumanized (being used like a horse) and being physically controlled (by bit, crop, etc.).
I don't generally get into the mindset of "I'm a horse", but that's actually a big part of the allure for me: wearing hooves, tail, and ears, I can be made to look like a horse despite my mindset to the contrary. Likewise, wearing a bridle, harness, and bit, I can be controlled like a horse regardless of whether or not I see myself as one.
Indeed, when I'm tacked up, I'm the only one who knows I'm not in a pony headspace (I mean, you do too now, of course, but you see my point). To anyone watching a scene, I would look like a horse (well a human horse at least), be treated like a horse, used like a horse, and act like a horse (unintentionally, of course: but simply trying to avoid the crop or resisting any of the bondage actually ends up looking like an equine mannerism weirdly enough), yet in my head I would still be a human.
Moreover, in addition to my mindset, I identify as a bondage (and sometimes show) pony, so the equipment I wear is a huge part of the experience for me from that perspective as well. As a bondage pony, the physicality of the bondage is often integral to the scene.
Similarly, the "costuming," or being made to look like a horse, completes the psychological thrill of being transformed into a human equine. I love looking into a mirror and seeing a horse stare back at me, or feeling my tail swish against each leg as I'm being handwalked.
As a quick aside, adoring tack doesn't mean you have to spend a lot of money on it. Nearly every piece of bio-horse tack can be used on a human pony with little to no modification. Pick up a used bridle on ebay and convert it to fit the human head, or grab some Davis boots to turn hands into hooves.
Seriously, there are all kinds of pieces of bio-horse equipment you can use to restrain and control your ponyboy (believe me, I've been in the bio-horse world for many years and I still come across pieces of equipment I've never encoutered before). Being obsessed with tack doesn't have to break the bank.
However, an equipment-centric attitude is certainly not the only way to approach pony play; in fact many ponies tend to view tack in a "less is more" way, focusing instead on emulating the behavior of an equine. I think this point sometimes gets overshadowed by all the awesome gear available. After all it's much easier to take a photo of a bridle than a pony headspace, yet both are equally valid ways to play.
Where you stand on equipment will depend on the mindset of you or your pony and what style of play you're exploring. The ponies that fall into a "pony headspace" where they see and interact with the world in a manner similar to a bio-horse may not require (nor want) as much tack as those ponies who keep their human persona during play.
Thus, it is important to work out with your trainer/pony what amount of tack you are comfortable with and what you are trying to achieve by using the equipment (e.g. are you using a bit solely to guide a pony, or also to prevent coherent speech; the answer to this will dicate the type of bit used).
Regardless, don't let others define your play. I enjoy using lots of tack and equipment, other ponies I know prefer a simple bit and tail. Both are totally fine, it just depends on what you enjoy.