Marriott has never "discounted" their TP awards. Though as some have already mentioned, they do periodically throw in airline bonus miles.
A couple of caveats for you if you are really thinking about ordering a TP award:
1. Carefully decide which airline you want to transfer into miles. Once the points have been transferred, there is no going back. (Yes, you can return the hotel portion, but you will only get a very small amount of points back.)
2. While Marriott has in the past offered up to 50% bonus miles for certain airlines, they haven't done that in a few years (and I suspect they probably won't any time again in the near future.) Note that I'll admit I am guessing with this, but given most all rewards programs are cutting back benefits, not adding to them, I'm not bullish on Marriott doing another 50% bonus anytime soon.
3. On that note, there is no guarantee that Marriott will offer more "bonus" miles for TP awards in the future. Yes, they have the ongoing United bonus and that will probably stay around as it is more of a permanent bonus. But there is no guarantee that next fall Marriott will partner with American and offer another bonus. (They may well do it, but there is no guarantee.)
4. You will normally get the most "value" out of a TP award by maximizing the miles transfers. So normally one would want to transfer the max number of points into miles if possible (i.e. for most airlines that would be 120K miles.)
5. Depending on where/how you want to use your miles Southwest might be a good option. If you transfer more than 110K miles into Southwest Airlines, you will not only get the miles, but you will automatically have the miles needed for a Companion Pass (CP). That means you get to designate one person to travel free with you on virtually any Southwest flight (cash or points). This has the possibility of effectively doubling the number of miles you get as your companion is flying free. And the big kicker with this one is if you do the transfer early in January, you will have the CP for 2 years. It lasts through the year you earn it plus all the next year. So if you order your TP award in January, 2016 with Southwest points, you would have a CP through the end of 2017.
6. The hotel award in a TP doesn't have to be used when you use your airline miles. (The airline has no knowledge of your hotel award or where you might use it.)
7. It is fairly easy to "upgrade" the hotel award from a TP award. So if you aren't sure where you want to use the hotel cert, order the lowest category (1-5) and upgrade if you end up wanting to stay at a higher category hotel. Downgrading a cert is much more of a hit or miss proposition. I wouldn't expect to be able to do that. So order low and upgrade if you need to.
8. Don't forget, if you don't have the points for the TP award you want, if you know anyone with extra Marriott points (spouse, family member, friend, etc.) Marriott allows a points transfer of up to 50K per calendar year per account. And for an award, they will allow a one-time per year per account transfer of even more than that. So you might well be closer to a higher category TP award than you thought.