I know they’re expensive spells, but the ability to add dimensional anchor to your binding is pretty amazing. The fact that it is instantaneous and thus permanent, making it reusable, is also a good quality.
I think it could be worth it if you plan to bind often and have a single location that you will often be binding in. The thing that makes it less juicy is the fact that only one other spell effect can be applied (you mentioned dimensional anchor, the best one imo but something like bane could help you coerce outsiders by making them more prone to fear effects; maybe that's too much though). So, in my humble opinion, it is entirely determined by your character whether this is worthwhile or not.
If I planned to have multiple planar visitors I would use it, if I was afraid that they would run away.
Can you also apply piercing metamagic to these spells? This can help with spell resistance? Should I worry about the 40 ft. radius limitation?
1) This is actually a fine idea, and I completely missed it in my Guide to Planar Binding. I'll add it next edition, if there is a next edition. (If you're looking for particular monsters to bind, search "crowdsource guide planar binding" -- there are three threads. Also of possible interest: "DMDM's Miniguide to the Gate Spell" and "DMDM's Guide to the Diabolist".*
2) As a practical matter, this is insurance against that 1 time in 20 that you roll a nat 1 on your Charisma Check and the outsider busts out of your calling circle. It's only going to be an issue with ousiders that can teleport or plane shift. Which is a lot of them, but far from all. You can get some pretty powerful elementals and whatnot that don't.
3) I don't believe there's ever been a ruling on whether you can apply metamagic to the spells in a Hallow / Unhallow. Personally I'd say no, because that can get OP in a couple of different ways. Don't forget that if you can use it, the DM can use it against you... that said, I don't think it's very clear under RAW, so if you can convince your DM, go for it.
*Most recent version, Autumn 2015. The Diabolist-specific stuff has been rendered mostly obsolete by the nerfing of the Diabolist in Book of the Damned, but it has a lot of stuff that's relevant to Planar Binding generally.
Doug M.
I was more interested in applying metamagic to Hallow/Unhallow itself. Going meta-meta with the “internal” spells does seem like a recipe for munchkinry.
This idea confuses me. Why would you dimensional anchor an area you are trying to bring something across the dimensions to bind. The Planar Binding would fail automatically, or worse, it would appear outside the dimensional anchored area uncontrolled.