There is at least one reason why Brommie resale values are so high: Reliability. It's my understanding that component fail rates are fairly low on Brommies but fairly high on Dahon, with better parts support from Brompton as far as availability and delivery. I have no objective data to back this up, though, so pure speculation.
The Brommie is the ultimate multimodal commuter IMHO: Tiny fold, lifetime bike, incremental engineering. It does not excel at all things. Gearing can be limited and riding position can be wonky for some riders.