Squads are effectively useless because there is no mechanic for keeping them together. Individual members die and respawn, or branch off and mix with other squads, and it's just not practical to keep them in the same groups all the time. You can just as easily refer to marines by their location rather than a squad number. The only real purpose I can think of for assigning marines to squads is to easily give them separate orders after a beacon, but that's rarely necessary.