Derailleurs are one of the most abused parts of a bike once it is packed, one of the most broken parts upon arrival, and at least another 10 minutes to tune up after each and every travel. I'm done with them on folders.

One of the reasons I sold my Merc (and may consider buying another Curve or a Mini in the future instead of a Brompton or anoter Merc) is the absence of rear chain tensioner or derailleur. Trust me, it is just a matter of time to have a problem with one of those if you pack/unpack your bike often.

14R.