Probably the only valid reason for using
set enable_seqscan=false
is when you're writing queries and want to quickly see what the query plan would actually be were there large amounts of data in the table(s).
M. Variščić. Udruženje za zaštitu okoline Zeleni - Neretva ; Fondacija Heinrich Böll, Regionalni ured Sarajevo, Konjic ; Sarajevo, Third издание, (2011)