Possibly soldiery will disappear and be converted to poetics, but most likely poetics will remains as they're 10 levels below the new tomes and are a part of ARR content (I'm not sure who would do this, but I'd imagine there might be some people who don't buy the expansion but still play and they'll need something to earn).

Since poetics are likely here to stay (at least for now), spending them now or later makes little difference as long as you don't hit the cap (if you're in danger of that weapon tokens for new or future jobs are a good investment). New crafting items will probably be added to poetics that you might want, so if you more or less have your fill of gear that might be something to save for. It's unlikely that poetics will see any new equipment or conversion into another currency though.