Obviously I'm not in a position to comment on anything specific to fraternities.
But my chapter recruited, and still recruits, year-round. We've had one-person new member classes. We always tried to give even those small NM classes the full new member experience, including I-week (which also serves to bond the whole sisterhood together) and initiation - often, where a NM class consisted of one woman, her big sister would participate alongside her little as if she were a new initiate herself.
So your spring pledges miss the fall activities. They will get to experience them next year. If you want to pledge these men who come through in the spring, snap 'em up now - don't let them miss out on spring semester activities, and don't watch them go to another fraternity come next fall.