Is it really that important to know how much your bike weighs? or am I missing something...are you going to use this type of scale for something else on a regular basis?

Once you know how much your bike weighs it aint gonna change unless you change something on the bike. You should be able to estimate the weight increase or decrease. Unless you get some sick/stupid boost to your ego by bragging to your riding buddies about how light your bike is, down to the last gram, then you can avoid spending any of your hard earned cash on this issue.

Take your bike to a shop and have them weigh it and be done with the matter.