I suspect snobbery plays a part. Although it does add weight if you go too mad with it.
The problem is not with filler, but with the less experienced who use it incorrectly. The idea is that filler is used to make good imperfections, it is not a replacement for repairing the panel to a reasonable standard first.
Could you elaborate to when it is acceptable and when it isn't to use filler?
Good = 1mm covering imperfections after panel repair
Bad = 1cm in lieu of panel repair
Filler should be used as a means of surface preparation for any imperfections that would affect the paint finish, not to repair holes in panels as some people think ( think of a hole in a wall, you wouldn't fill it in with filler, you would use bricks , then plaster it. Then , if necessary , a touch up with filler to get the final finish good enough for paint ).
Are you actually being serious?
Why must it not be used to repair holes in panels?
Honestly, does it really need explaining?Why must it not be used to repair holes in panels?
Cosmetically because it can and does crack and fall out. Metal has elasticity that the filler can't replicate.
Structurally because it's just not as strong. Sure, an inch thick wodge of filler is difficult to snap, but it's just stuck to the metal around it.
"But there are cars entirely made of composites and they're strong enough" - they're designed to be made like that, a metal panel isn't designed to have epoxy or similar sections.
(I am wondering though, why did you join a welding forum to enquire about every way possible to avoid welding and metalwork?)
Because filler is porous and allows and holds water against the metal around the hole and it continues to rust away unseen, until one day it falls off and you're left with a hole considerably bigger than the original. It's OK if it's used for its intended purpose of covering minor imperfections but it get a bad reputation by being used by the moderately skilled to make POS look presentable.
Colin
That is correct, but you would still use a skim which is flexible, rather than a tennis ball size lump.