I accepted an Aer Lingus voucher on a similar basis re extra value. It does have an expiry date, but I travel to Dublin regularly.
I have EasyJet cancellations in the pipeline. If they offer me that deal I'll be happy to accept. The open date thing is far more important to me than the value hike. It can't expire on me! And I'll definitely fly again when allowed to do so.
The only concerns with schemes of this type are whether you'd have any cover if the airline concerned ceased trading. And with a foreign airline (eg. my own booking with Air Baltic) if the carrier cancels plans to launch the new route you had been booked to fly on. Don't want a voucher for a carrier which no longer serves your route of choice.