Simply put, most players I have talked to like the idea of sideboards at first, but as debate heats up, no one likes it.
One of the strengths of Jyhad is that each deck lives up to its own merits. Decks play against each other on a relatively equal footing: they either have strengths or weaknesses inherent to the design that makes them at an advantage or disadvantage. The key is that it is by design, and typically a part of an overall conscience strategy choice. While someone may include a Letter from Vienna in their deck, it is not common, and usually only done if a player knows that Stereotyped-Tremere (either Intercept-Burn or Bleed-Burn) will toast him.
Sideboarding changes that.
Rather than making intelligent deck creation decisions, players will instead fall into the Magic frenzy of (and apologies to non-Magic players) "Land destruct, eh? I'll throw in my Lodestone Baubles" or "Turbo Stasis? Time for the Ankhs of Mishra!". These cards do nothing for the deck and merely exist as a 1-card answer to another player's deck design.
Jyhad sideboards will likely consist of 1 card to hose each of the most heavily played clans (Malkavians, Brujah, Gangrel, Tremere), a couple to hose specific strategies, and one or two more to screw over a clan or strategy that is slightly more difficult for a deck to deal with. Simply put, thought goes away, and the game becomes almost similar to Jyhad-Euchre or Jyhad-[insert trump game here}. One card that really had little or no place in the deck is used to destroy another player's game with little thought to design.