The basic problem is the logarithmic curve of loads on the spar: insignificant at the top, but steep at the root. Traditional strut-braced wings tend to be lighter, but external structure reduces top speed. If you look at the World War 2 vintage Horsa glider, you will see mostly cantilever wings with a small diagonal strut far onboard. Struts allow for a much simpler wing spar, maybe even a constant chord spar. The disadvantage is that plank spars are always heavier than tapered spars. In order to be strong enough at the root, they end up with too much strength/mass/weight near the tips. There are a variety of ways to taper spars: lightening holes near the tips, varying chord, varying spar cap depth, nesting multiple spar caps, laminating extra layers to the front and rear, etc. With enough internal taper, you no longer need external struts.