I think usefulness of a tach varies by bike. For example I have a Bonneville and a VStrom 1000. The Bonneville has a very flat torque curve - it is very forgiving about what gear you're in. I've had the bike a couple of years, I shift by ear or speedometer reading, it's never been a problem. If I add a tach to that bike now it will just be 'cause the Triumph combined tach/speedo looks better than the basic speedo unit that comes with the bike.
OTOH, a tach is very necessary for the VStrom. The bike needs to be kept above 3K rpms if there's any load on the engine, and it also has a pesky flat spot I haven't sorted out yet at 37-3800. To do any serious acceleration, you want to downshift to kick it up above 5K rpms. I use the tach constantly to manage the engine on the VStrom.
A tach isn't really necessary to ride, but it is when doing mods or you want feedback for whatever project you are working on with the bike. My bike is tacholess and I can ride just fine. But for upgrades and whatnot, I would like to see one.
jstark, the Duc is the same way below 3-4k. It hates rpm below that and likes to lug. I hear it is "common" but the DP Performance kit for $1500 more wil lmake it run like a normal bike should
