Any FMS more than 500m from target already gets berated in chat by players spawning in, for being "too far" from target, and they then despawn having jogged far enough away from the FMS to waste the unit as MIA.
I can see Xoom's point, though I am not sure that it should just be a perk of HC and RATs, especially when the UP side probably sees 6 or 8 hours per day without either in game (more in the final days of a map). There are only a few available, so make them Rank 10 spawnable, and leave them at a handful per flag/garrison. Are the numbers rifles/smgs available to be spawned limited? I seem to recall reading that the HC walking MS was limited to just 17 rifles?
The biggest issue is that 20 players can work together to set a ZOC, either on Attack or Defense, and one "HC" can wander off on a solo run, and sneak in silently behind the ZOC, plop down his magic beans and then up pops some smgs and rifles to undo the hard work that the team has put in to set their ZOC and protect their FMS - 4 rifle satchels, and bye bye FMS. (FYI I think 4 is probably about right, possibly make it 5 to require more then one engineer, but in an attritted flag, having no engineers left in supply, it gets to be a ballsache getting a EFMS down, even without the EIs spawning in.