If it is not stated in their T&Cs then that comment above is utter rubbish (no disrespect to yourself obviously)
Yep. Say you had the following flights booked from Manchester Airport:
12:00 1st May Manchester to Porto
Porto to Manchester arriving 02:00 7th May
06:00 5th Oct Manchester to Tenerife
Tenerife to Manchester arriving 18:00 19th Oct
You might need a taxi to get home after getting back from Porto and to get to the airport in time to check-in prior to going to Tenerife. But if you want to use the train to get to the airport on 1st May and back on 19th Oct, I don't see why you can't use both parts of a complimentary ticket for that purpose. It doesn't have a 1 day/month expiry like a standard return ticket might have. Even with a standard ticket you can choose the date of return after buying the ticket, the date printed on the ticket is the last day you can travel on it.
Do they not still do this for serious delays?
While the tickets have an option for 'all day', the usual practice is to issue 2 x complimentary tickets for delays of over 2 hours, not a ticket for unlimited all day travel.
If you're out hiking and the walk start point is on one line, while the end point is on another line then a complimentary return isn't much use.