I think, as noted above, the one downside to doing this is that you might end up not being able to take advantage of lodging package deals from disney.
Room only discount codes seem to be disappearing. By far, the best savings these days, if you stay on site, appear to be the package deals that combine room savings with ticket savings, or give you free plus or hopping features on your tickets. Moreover, many people like the packages because they have the dining plan. It's hard to take advantage of these packages if you already have prepaid tickets. At a minimum, you need to end up buying at least one day of tickets (which, for a family of 4, can be well over $200) in order to get the package. So, you're in position of either paying non-discounted rates for rooms or taking those prepaid no expire MYW tickets and sticking them in a desk drawer for a later trip.
We like to stay on site, so this is a downside for us. We're planning a trip for later this year and feeling a little foolish that we're sitting here with never expire tickets that it looks like we won't use because it's not cost effective to use them. If you stay off site, by all means. But if you stay on site -- and I know this is not consistent with the advice you read elsewhere -- I'm really starting to believe that the best option is simply to take the best deal you can find on what you need when you need it and just trust that another good deal will pop up in future years when you need it.
(Now, this is a sheer financial analysis -- none of this addresses the psychologicial aspects of never expire tickets. For some, perpetual planning and a drawer full of no expire tickets has a psychological value in addition to a financial value. It's just darn nice to know you've got a prepaid admission for that next trip. I expect that many of us, since we're here on this board, are in this category. In that event, have at it! I wish they sold a 100 day no expire ticket!)