Those type clamps can work just fine, but they do leave a hole where the points grabbed the wood.
Not that one couldn't use them for crown or baseboard molding, but why?
Fit the trim well, and install. If paint grade trim, caulk any gap (if pretty minor, you can 'burnish' over the gap to close it up) and then paint. If stain grade trim, put wood filler/putty any gap and then stain and varnish.
If you want to use some glue on the mitered corners (outside corners ONLY, inside corners get COPED!!!), use a 'thick' glue (Titebond makes a trim/molding glue made just for this type application
http://titebond.com/product/glues/a1e18a48-c721-4b0b-8f63-259c477919e0 ) and a thin coat and then make a "rubbed joint" (literally rub the two pieces together to spread the glue and get it into the wood pores/fibers better and when the glue 'grabs' you know you have the beginning of a good glue joint).
Or you just a pin nailer to shoot a pin across the joint to hold the joint together while the glue dries. Could maybe use a brad or trim nailer, but that leaves a small hole from the head of the brad or trim nail that has to be filled (caulk or filler/putty). The pin nailer hole is so small you usually don't have to do anything.