The only things you need to pay for are skins. Everything else can be unlocked by playing.
A few things. If you buy even the cheapest pack there is, you get +50% XP for life. I think if you like the game giving just a little to the developers makes sense. The cheapest packs is €7.99 (not sure how much if you buy in $, probably 9.99). If you buy a booster pack you get another 100% XP on top.
So, somebody who pays nothing unlocks things 3 times slower than somebody who gets a booster pack. Doesn't seem over the top to me. Sure somebody can unlock everything by just paying for it but tbh it takes some of the fun out of the game (for me at least). Very few things can be considered an absolute must to unlock. You can play very well with only the initial 3 free classes with a few unlocks.
Anyway, now as the leader of a Tribes: Ascend clan/guild I'm going to try and recruit you. Hope you don't mind :P
The clan is called Silicon Warbunnies and we are a part of an international multi gaming community called The Conclave (10 years old). We support multiple games from Tribes, to BF3, WoW, LoL and others. We are a mature community where you will find very little drama. We use Team Speak 3 to communicate while gaming.
If you are interested then visit us a
http://conclave.cc or feel free to PM me for more information.
Forgot to mention, most of our Tribes members so far are from EU and play on EU servers. We do have two or three who play on NA servers and would definitely welcome more.
Thank you for your attention,
Aningan.