Looks like I'm pretty late chiming in on this, but these days I rarely duck my head out of the editorial boards.
There is certainly a 'fan' base out there for harlequins. There will always be those most interested in such a secretive and esoteric sect, who is probably the most rare (by volume if nothing else) faction in existence (aka other than those fully 'extinct' as it were, such as squats, zoats, etc.). The problem is that aside from the new codex entry, most 40k players weren't around as far back as 2nd edition to be familiar with them. I used to field my harlequin army in days passed to much enjoyment, and acceptance among my gaming mates. As time went on I ended up using the codex approved rules here and there. But as things are now, unless people already have a harlequin interest from back in the day they don't likely know about the potential options for harlequin armies today (not to say that they are balanced and cohesive and all that, but that it's a possibility for friendly games). I think that it is certainly a possibility to develop them and make them the elites and HQ of an army where in a Chaos Cultist type fashion Eldar and Dark Eldar units are used for the meat would be potentially capable and cohesive, but I'll ignore any worthwhile word concerning the difficulties and efforts involved in actually wanting to trying to make such an official codex work. It'd be nice just to have some current 'allies' rules where the use of DE and CWE units were sanctioned officially and the fans and players could find what worked for them.
Alas...pipe dreams as it were in large part. I do agree that it is very unlikely to see Harlequins in any DE dex in the future, for the reasons Rasmus cited, but hey, we never thought harlequins would see the official light of day again too, so who knows the possibilities.
As for the aesthetic, to each their own. If some one does hit them with a blast of tie die...part of me does wither a bit inside, but there are good ways to do harlequins. One must also not forget from a fluff standpoint their holo-fields. They are a blur of prismatic colour, and remember to envision the changing masks. Harlequins are maniacal psychotic monsters to their enemies of such an exotic and alien nature, even to the Eldar standard, that I think a comparison with a stereotypical clown is an effrontery.
I think due to the time since Harlequin armies marshaled that for most fans it is but a matter of a fond memory, with little conception for a future, and those of the current era only know of them as a detachment. Regrettable, as I think that the Harlequins are really what hooked me to Eldar, and here I am what...18? years later still playing Eldar (theoretically anyway, haven't played a game with my Eldar but once in 5th ed.).