According to this question, if I'm going to buy packs, I should purchase Veteran packs first and Spectre packs afterwards, and that's what I've been doing: as soon as I hit 20,000 credits, I blow it all on a Veteran pack.
Should I be doing this? Is there somewhere else that I can spend money that I haven't found yet (such as extra medi-gel or something), or is buying packs the only thing that credits are good for in multiplayer?
The only thing you can spend your credits on currently is packs. But as the current special equipment pack shows, Bioware might occasionally add some different packs from the standard ones, and those packs might allow you to get certain equipment faster as they might be less random.
As you can see in the description, this special is time-limited.
So in general there is no use to hoarding credits, you'll only spend them on packs anyway at some point. The exception is if you're speculating on Bioware to add some interesting packs in the future, and want to buy lots of those when they appear. But unless you have some inside information, that would be pure speculation and they might never add more useful things to buy.
If you have the most important equipment for the classes you're playing, it might make sense to always keep a certain amount of credits on hand, so that you can quickly act if Bioware puts out an interesting, time-limited pack.
There have been cases where a time limited pack have been produced, such as the elite specter pack for 99000 credits that guarantees 2 rares and increased chances of rares. In these situations, its more beneficial to purchase than the normal specter pack and thus I think its worth it to save up credits to take advantage of these special packs.
When first building up your stock of weapons/chars/mods, it's better to go with vet packs. Once you have aquired all the rare (Gold) items and are chasing the ultra-rares, it is more cost effective to hoard credits until the time-limited Premium Spectre packs become available. These have twice the chances of getting an ultra-rare as a normal spectre pack, but only cost 39k more (99k for premium, 60k for standard).
There is no use to hoard credits; The only use for credits is to buy equipment packs.
Currently, Bioware has a special "Equipment pack" for 20k credits that will only contain consumables, but it's temporary, and it is unclear if it might make a regular appearence.
