Technically you can but you would loose a lot of value with the points.
What I meant is that they can't re-deposit those points back into your Marriott account like the can with the hotel certificates.
Actually, there is fractional value in doing so if you stay at a hotel for seven consecutive days.
Buy a Cat 5 Cert for 270k MR points and opt for United Airlines miles which gets you up to 132 UA miles.
Transfer those 132 UA miles back to MR and receive 132 MR points.
So for 7 Cat. 5 nights you essentially paid 138k MR points.
If you reserved the hotel with MR points outright it would have cost 6*25k MR points = 150k MR points (taking the 5th night free into account).
This way, you can save 12k MR points.
The 12k savings applies to all categories from Cat. 5 upwards.