If your stay in Brussels is no more than 1 night in each/either direction, you can use ABS fare option anyway, and not only if you decide to stay in Brugge - when you would still need to respect the "next day" rule as regards transfer in Brussels, in each direction! ABS costs about €7 on top of normal London-Brussels/Brussels-Paris fare, compared with €13.50 from Brussels-Brugge if you buy this separately (standard single: weekend return is only €14.40 so little need for ABS rate if doing this trip over a weekend).
Effectively using Eurostar ABS ticket, you have a ticket London-Brussels plus Another Belgian Station (Brugge), as long as you depart Brussels no later than the day after arriving there on Eurostar; and if returning to London, a ticket from ABS destination (Brugge) to Brussels, where you can (again) stay for up to 24 hrs, and then Brussels-London no later than the following day. If not returning to London, but continuing to Paris on Thalys, there is also a Thalys ABS fare which operates in the same way, i.e. it permits up to 24 hrs in Bxl after starting trip to Paris in e.g. Brugge.
Thus if you stay in Brussels for 2 nights, you arrive in Bxl on Day 1, and would need to depart to Brugge for day trip on Day 2, using Eurostar ABS ticket (covering London-Brussels (overnight) - Brugge). Then when returning to Bxl from Brugge, on Day 2 (if spending that night in Bxl), you would use your Thalys ABS ticket, also using this to continue the following day to Paris.
If not spending that night in Bxl, then of course you can still use your Thalys ticket at any later date, it is not your stay in Brugge that is limited but the time you spend in Brussels on stopover ("VIA" destiination - you would be travelling London-Brugge via Bxl and Brugge-Paris via Bxl).
Personally I would stay in Brussels, near Gare centrale, in order to reduce humping suitcases on and off trains. But you are the only ones who can decide whether you want to be in Brugge in the evening. Although even then, trains continue from Brugge to Bruxelles-Central (calling first at Midi and subsequently at Nord) until late: the last trains on a weekday are at 21:58 (arr. 23:00), 22:32 (arr. 23:57) and 23:05 (arr. 00:09).
Edited: 8:09 am, November 18, 2012