Hm, as far as i know, you only can use select worm
after touching all walls. The rules say you can only attack after touching all walls and having collected a weapons crate, as select worm
is in a weapons crate i would consider it to be a weapon.

Tactically it is one, definitely.
To make it more clear: Using SW like that rapes the concept of wxw. You could theoretically make 4 walls in much less time than using only 1 more. To be precise, using SW after you touched all walls also rapes its concept as you dont have the way back to attack, for example. That said, SW should actually be completely forbidden for wxw and/or shopper.