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).
Y. LeCun, F. Huang, und L. Bottou. Computer Vision and Pattern Recognition, 2004. CVPR 2004. Proceedings
of the 2004 IEEE Computer Society Conference on, 2, Seite II-97--104 Vol.2. (2004)