Kinda starting to swim in them, but haven't really figured out how to spend them on anything useful. What they're for?
I never use mine except when crafting for a legendary. I've got thousands saved. Id have even more if i used ToK .....have like 5 stacks of those
Spritishard => Gold (powered by gw2efficiency.com)
I would not use/waste all of them. Especially lengendary weapons need certain materials purchased from the Mystic Forge Attendants (aka Miyani in Lion's Arch). Anthology of Heroes is used to swap stats of ascended gear. Aguru Stones are needed for ascended crafting. But as you can see, in low amounts. And you constantly get new spirit-shards anyway.
Don't worry too much about spending them. Shards are one of those items/currencies that players feel they have far too many ... or far too few. Hardly any of us hit the Goldilocks sweet spot of just enough.
@HnRkLnXqZ.1870's link is spot on: you can convert each shard into 50-100 silver without much risk. (However: it is tedious work.) Done well, it's more efficient than farming Istan or Silverwastes (but it also puts more people to sleep).
"With great power comes not-so-great utility bills."
I have 8,500+ Shards currently. To convert these to gold would require too much patience, and/or a lot of wrist pain to upgrade mats to another tier, so I choose not to. It's far too tedious a task. It's not really worth the hassle when there's better ways to make gold in game.
Don't worry about them. You'll have as many as needed if you choose to make some bigger ticket items that require Bloodstone Shards or Eldritch Scrolls from Miyani.
The above was written as part of an attempt to waste time.
I wish I had that problem. You need them to buy things like philosopher stones, which allow you to infuse rings. Also to make ascended gear, or to convert stats on ascended gear. With any currency in the game - hold onto them until you know what you are going to use them for. Wasting them is a bad idea.