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By removing the spouts, you remove the metal that would otherwise knock some of the air out of the crema. This means that you get less liquid in the resultant shot for the same volume that you would extract with a regular portafilter. You can confirm this by extracting a two 60mL shots in the same amount of time; one with the bottomless and one with spouts. If you let them sit side-by-side, you will notice that the bottomless shot ultimately collapses to a lower volume than the regular shot. This means that if you are extracting the same volume with a bottomless shot, you are actually extracting at a higher brewing ratio, which means that your shot is closer to the ristretto end of the spectrum and, hence, tastes richer. You could achieve a similar result with a clean spouted portafilter by simply tightening up your shots. As to the relative merits of espresso vs ristretto/lower brewing ratios vs higher brewing ratios - thats a matter of personal preference.