I still use them and carry them, in addition to glass filters, both of which I use and recommend, especially since I shoot several formats, including Hasselblad and Minolta 6x6cm, 135, 120 roll medium format, 4.5-6x9cm, 4x5 and am building a 5x7" pinhole.
I believe new shooters should invest in a full set of colour correction and ND filters to begin with, rather than the expensive glass filters for a growing group of lenses, other than polaizers, which I feel should be good quality.
Adjustable square gel holders are out there, look first to eBay, for the spring tension type which will fit a reliable range of lenses/formats.
Simple methods abound for using these filters and as long as you take care using and storing them, they'll last years.
i suspect many more photographers here use them, so yes, there is a market for them.