I personally have never been motivated by a party alone. I don't even get how this would be supposed to work. How can a party be motivating just by itself?
What I clearly acknowledge though is that if party is not an instrument but a true expression of happiness and gratitude for whatever thing has been achieved in or by the company, something like management just have to share something because it's bursting out of them, it can have a very good impact on the team spirit, even to me.
In that respect I believe that people have a very good sense for whether something is just a tactical instrument or if someone is really trying to share something and honestly saying "thank you" for whatever it is.
I also guarantee from my personal experience that people are different. Some people like a barbequeue or a party in/with the company every now and then and see it as something that makes their work life cool and motivates them, others see it as a waste of time and would rather like to get something done at their desk or spend time with the family instead.
Most of all you should never forget your own mentality. If you're not a party person, it's unlikely that your parties will excite those who like parties. You probably have more authentic ways of encouraging and motivating people in that case.
If you're unsure, how about openly asking the project team what would be a reward or thank-you they'd be looking forward to when the project is finished?
Years ago worked in a company that did super-expensive parties and were quite proud of it, but the salaries they paid were below average. It always gave me a feeling of: They throw an expensive party that makes everyone feel like royals, where they spend, say, $500 per person, which gives a shiny touch to the job, but effectively the company just saves a lot of money as opposed to simply paying people properly and really making long-term commitments with it. The expensive parties looked so generous and grateful on the surface, but to me there was a bitter taste about it.
Bottom line: I think it's not the act that counts, it's the true emotion behind it that counts. If that's positive and authentic, go for it. If however you feel weird about it or it's purely tactical in the first place, consider something else or not doing it at all.
If team motivation isn't good, look for the real reasons, simply more parties might barely patch it up if at all.
That being said it's becoming customary in many companies that employees start expecting their workplaces to be an area where work and leisure time melt together. Even if you don't feel personally inclined to follow that trend, notice it at least, and consider it a valid factor that might encourage people to work for you and do their best for you or not.
Nonetheless they're all individuals, in the end you will need a solution for those individuals, not some textbook thing.