I disagree....

I have an aftermarket tach and it is more accurate than the stock tach...

at about 3000+ rpm the stock tach is off by almost 100 rpm's... it just gets worse the higher you rev.

So yes it actually does have some value... being a more accurate gauge than stock.

As for there being a need for it... well, no, not really... unless you really have a need to accurately gauge the rpm's....