Hello Reddicediaries,
I agree that dissolve is a bigger problem today then it was last year (mostly because of the siren), but you need to remember the reason for the battle forge in the first place. Most of the time the Battle Forge is meant to cast low cost equipment (low chance of being dissolved), or it could be meant to cast higher cost replacement equipment. In either occasion you are still saving actions and possible mana with the battle forge.
The Battle Forge itself often cost more to destroy then it is worth, the exceptions being: Ballasta,Swell(sometimes), Darkpact Slayer, ect.
So I don't see the dissolve thoughtspore as a problem. The acid ball thoughtspore is more problematic, but even then you could just cast rain cloud.
I would not cast a Battle Forge as a Druid, Necro, Beastmaster (depending on deck style) and Wizard.
The SP for a BF is just too expensive for the Druid. The Necro and Wizard usually try to avoid combat, and often have better things to spend their mana on. The Beastmasters are in that weird inbetween state, it cost 5 SP, which is a little too costly (personal option), the nature school also has some of the best enchantment armor, so...... it depends on your deck style.
BTW: I'm try to finish a "Best opening for Siren" post, so if you have any experience with a Siren + Battle Forge opening, I would love to hear it.
Thanks
Werekingdom