He means it could be a very good deal to buy miles for 1.7c if you redeem them in the right way. Say your RT business class ticket (which you are buying anyway) is $5k. If you can get it for 120k miles and you only paid 1.7c per mile, you are getting the ticket for $2k instead of $5k. So that's a huge win.

On the other hand if you aren't able to use the miles for that flight and you only take trips like that one every year or two, the miles may sit in your account for a long time, the award charts may change, and by the time you get around to using them you may only be able to redeem at a value of 1c, so you would be losing money, not to mention giving AA an interest free loan.

So in general you don't want more miles than you can use in the foreseeable future. I find that having a 1 year buffer is helpful for flexibility, but anything more than that is essentially useless and taking unnecessary devaluation risk.