I can only think of 5 times I have intentionally "spammed" trash for benefit.
1. Apprentice. +1 action and the ability to spam draw cards means that multiples of this are not dead, but that late game +5 cards can either enable mega turns or allow me to drag a dying deck along for just one more province/duchy. The key here is +buy and a reasonable way to build value. Common enablers are things like hoard, highway, bridge, transmute, crossroads, ironworks, haggler, etc.
2. Upgrade spam. I don't think I have come across a case where upgrade is good and where more upgrades aren't better. You can use them to trash out garbage quickly with no collision worries and only play an effective -1 card when you do. Once you have taken out the trash you can either start in on the silvers (normally upgraded estates) making throne rooms, spies, or such ... or you can start trashing upgrades with upgrades (I'm a huge fan or trashing them for nobles, harems, massable goons, and border villages - the last being perfectly fine to do for upgrade an upgrade -> BV + upgrade sometimes).
3. Transmute is a difficult card, it is a pretty poor trasher and it is so very slow. That being said, when it is worth it (e.g. venture) I normally end up spamming the buggers. While slow, I can get some gold and then turn the coppers into "potential duchies". This is rather powerful late game when there is no other way to hit duchies. Also this is extremely good in a duke game where you can buy silver (or even copper) after you have done a quick duchy rush to help empty out the duke pile and crunch coppers (and later anything) to empty out the transmutes.
4. Scrying pool. If there is nothing else good in the 3/4 range I may gain a bunch of trash for benefits. Getting rid of coppers & estates means I can keep drawing. Something like village/transmute/secret chamber means that mostly useless transmutes make for cards I can draw/discard multiple times and late game maybe burn into multiple duchies.
5. Border village. Remodel a remodel to a border village and a strong 5. This adds +action to my deck and gets me a five I may want more than gold e.g. minion.
Most of the other trash for benefits situations stall out if you overbuy. Sure I'll double up on expand or remodel, but if you go heavy enough you:
A. lack support cards
B. don't get to thin out your deck
C. have triple collisions - remodel/remodel kinda works, but remodel/remodel/remodel leaves you with a dead 2 card hand.
D. May delay effective game end - one of the charms of cash for benefit is that you can hit provinces to speed up endgame and diminish the points left the other player can nab. I.e. If I'm holding province/silver/silver/copper/remodel I can buy a duchy and trash a province into a province. If I'm holding province/silver/copper/remodel/remodel I can remodel a remodel into a duchy xor a province into a province.
Long story short - generally stockpiling trash for benefit isn't good without high +actions (either on the specific cards or in your deck) or a special mechanic (transmute a copper -> transmute, transmute a transmute to a duchy; scrying pool & actions) that makes the cards valuable in a way that estates & coppers are not.