A dial or digital caliper is actually something that I don't own that I would like to get (you can get basic ones so cheap don't know why I haven't). I don't bother to evenly thickness my wood unless I have too for the piece's particular use, but there are times when it really matters and it would be convenient just to check with a caliper. Its not a requirement.... just check with my marking gauge or a stop rule and feeling if it is even around the board is good enough, but a digital caliper, while not necessary, would be handy in some cases. A lot of the tools we talk about on this forum aren't required, but that doesn't mean they aren't nice to have. Would also be nice for say, checking auger widths against dowel stock maybe....I could even see if being used for checking fits of joinery in some case - not to dial an exact numeric fit, but maybe just to get an idea of how far off things are. Another place it might be useful...sliding dovetail...ever been fitting a sliding dovetail and trying to figure out exactly where its off. Its not always and issue of being just to fat, sometime the angle is a tad off and by measuring both the widest and narrowest part of both the male and female parts of the joint you could very quickly tell if and where things are off (of course dividers will do this for you as well). Anyway, would I actually use a dial or digital caliper this way in my shop? Maybe, maybe not....don't know. Just kinda reflecting on ways in which one might be handy. For me it probably wouldn't be used to check all me tolerannces within .0001", but I can see it being pretty handy as a point to point comparison tool. For me the question as to whether or not to use such tool comes down to whether they will make certain things quicker and easier and if the advantages they offer are worth he expenditure....if I think they'll lead to lots of fussing I'm not interested...if the have the potential to help with a few things here and there, but are really expensive and not required, I'm probably not interested....but if they can make some tasks more convenient and aren't a huge investment its a worthwhile consideration.
I too really like the matched straight edge Kees and Stan describe. Since match plane my winding sticks in this way I use them as straight edges all the time. When its time to true them up, I clamp them together in the vise, plane a slight hollow, and then plane until I think that hollow is gone. If when I hold them edge to edge up to the light I see no light or just a small amount thats good enough, as the amount of hollow I might still be seeing is actually twice what each individual stick has in it.
Good stuff. Interesting thread.