Well, I have no idea what your costum designs look like, so I cannot comment on their usefulness.
The few canon designs we have (and I am talking strictly of modified Mechs here, not Industrials designed for combat like the Quasit or Arbiter) are interesting to play.
Designs like the Harvester Ant (MG) chew through infantry at frightening speed, and for 127 BV it is a bargain. The LRM version brings two LRM 5 onto the table for 215 BV- again, not too bad. Jumping to the heavier designs there is the PPC armed Buster Hauler Mech- fielding a PPC was never cheaper. The ATAE 70M Uni has two AC 5 and two SRM 4 and enough armor to be annoying- as long as no weepon beats the BAR of 5 :D.
My point is that armed industrials have their place and use, but cannot compete with a Battlemech one on one. Some have only BAR 5 armor, others rather inefficient cooling systems, their ICE engines might stall and all are more fragile, easier to crit.
Still I love them. Fun to play, especially in scenarios.