Darkice9: Holon's Castform,
Holon's Electrode,
Holon's Magnemite, Holon's Magneton, and
Holon's Voltorb all saw play.
Holon's Voltorb and
Holon's Magnemite might seem surprising, since they don't look too good.
Even back then a 40 HP Pokemon was small and easy to OHKO (though FTKOs were challenging but possible) and their attacks were mediocre, and their effect just let's them be attached (still counting as your normal Energy attachment) from hand to one of your Pokemon to provide :colorless: Energy. The key thing to remember is that running one (or even two) as TecH meant any Pokemon search card doubled as an Energy search card. It was a small but important benefit, though the risk of opening with one was still an issue. As such it was known but not universally used. There were also some "no Energy" decks that used them as well.
The other three have obvious benefit: while you are not gaining anything in Energy quantity (two attachments ultimately end up with two Energy), you're Energy quality skyrocketed! Even though these cards existed alongside
Double Rainbow Energy, they were played. Why?
Double Rainbow Energy provided two total Energy that counted as every type, but dropped the damage done by the equipped Pokemon's attacks to opposing Pokemon by 10 points before Weakness/Resistance
and was restricted in what it could be attached to: only Evolved Pokemon that weren't Pokemon-ex.
There were plenty of cards with multiple Energy type requirements, as well as being run in multi-type (or off-type) decks; for Pokemon-ex that made
Holon's Castform/
Electrode/
Magneton the only option and for a few others, was required if you didn't want your damage to drop below a certain point. Plus back at this point I believe we still had
Ancient Technical Machine [Rock], a Technical Machine that gave the Pokemon it was attached to access to an attack for
colorless
that bounced the highest stage Evolution card on
each of your opponent's Pokemon to your opponent's hand... which made
Double Rainbow Energy vulnerable to being discarded. I could be wrong though and there may not have been any overlap.
On top of it all, decks with Energy acceleration obviously liked
Holon's Castform/
Electrode/
Magneton, and that was one of the big combos for these cards. Even after
Holon's Castform was released,
Holon's Electrode and
Holon's Magneton still saw play if
Holon's Castform was an undesirable starter (it really depended upon the deck) or if the Pokemon count was such that it was too "expensive" to start with
Holon's Castform (the deck wanted all copies to be Energy and never Pokemon).
As a last footnote, while quite rare on select occasions the Energy bouncing aspect was quite useful, with creative Energy bouncing being as much an asset as a cost.