While emergency row seats may offer a little more legroom, there are frequent trade-offs;
- rigid arm rest / seat dividers
- no recline
- no drop down tray
- IFE screen that much further away, if at all
- no seat back pocket for documents, books, etc.
- no bags under the seat in front (added, with thanks to Capn Bloggs)
... don't see the value myself. Certainly wouldn't pay extra to use one.