Every time I've tried saving coin tokens, it's ended badly. Most of the time you want to spend it early on a card you want but can't quite get.
Here is my take on coin tokens:
In general in Dominion, you want to be building up your deck (duh), and the faster you get good cards, the faster you'll get good cards. That sounded like a tautology, but what I mean is that getting good cards sooner will make your deck stronger sooner so that you can more easily pick up more good cards. So the natural thought it "Why would I save coin tokens? If I spend them sooner, I accelerate faster." I think there are two main reasons that saving coin tokens can be a good thing:
1. The obvious and more common reason for saving coin tokens is because you hit say $3, and you don't particularly care for any of the $4's, so you don't gain much utility by spending the token now. In the extreme case maybe there are no $4 cards on the board, in which case it's a trivial decision to hang onto the token for later. In other words, they help smooth out buys.
2. The less obvious thing that saving coin tokens can do is they can postpone your greening. Dominion feels like a positive feedback game for a while, but when you need to start buying victory cards, it becomes a negative feedback game. In general, negative feedback means you want to not be winning for as long as possible, except at the end of the game when you want to win. If you could have your way, you'd prefer to never buy a green card until the game is over, and then buy them all at once. Coin tokens give you the option to do that. You can build up a deck that does things and gives you coin tokens. Just watch your coin tokens pile up and ignore victory cards and keep building up your deck to generate as many coin tokens per turn as you can. Then when you're ready, cash them all in at once and buy all the victory cards.
The trouble with this is that most boards just aren't the sort of board where you can do that. The reason why most boards can't do that is because you need to be able to build an engine that can generate so many coin tokens per turn that the money you can spend on what would otherwise be green card buys you instead spend buying more engine pieces, while still having enough coin tokens leftover to do all of the green card buying you would normally have done. In other words, you want most of the coin you generate to be in the form of coin tokens.
As an extreme example, imagine that instead of starting with 7 Coppers and 3 Estates in their deck, each player started with 7 Candlestick Makers and 3 Estates. If you just did nothing for the entire game except play Candlestick Makers, and then at the end of the game cash in for a big green card buying frenzy, you would hit 4 Provinces in 9-10 turns; 5 Provinces would take 11-12 turns. If your friend is just playing some normal BM strategy, using the coin tokens only to smooth out buys, then I have no idea how long he would take because I don't have a Guilds simulator. But it would probably be a lot slower.
So yeah, if you're considering going for a "build up a pile of coin tokens" strategy, you should ask yourself how high of a ratio of coin tokens to total coin you will be producing each turn. I'm not sure where the cutoff point is, but if it feels very high, then it's more likely to be a viable approach to the kingdom.
