Likewise as above, my playgroup allows the following proxies:
Proxy cards you own, in order to not need to swap between multiple decks.
Proxy cards you have purchased/plan to purchase, but haven't arrived yet.
Proxy cards you may want to purchase, but want to be sure of before making an investment.
The third is perhaps a bit more unique, but everyone has their budget, and some card might represent a fair portion of that budget. Sometimes you want to be sure the card you're thinking of will work the way you hope before pulling the trigger on a significant investment.
We've been talking about opening up our proxy acceptance though, in order to try to even out the playing field between certain players, but nothing has come of that yet.