Under what circumstances a full-time member of a project team is more preferred than a part-time one? And vise versa.
In general, a full-time member of a project will be better than a part-time member. The main challenge is domain knowledge and communication.
As a part-time employee, you may feel more left out from the team. The other members of the team will know details about the project that you missed because you were out when the announcement was made.
In addition, a part-time employee may have more trouble being productive, as it sometimes takes time to get into a rhythm, and there is just less time to devote.
If you have more resources on the project to make up the difference, you'll have to remember that there are still more communication paths to consider, different skill sets, and more distributed domain knowledge.
On the contrary, if the project involves many different skill sets where there isn't enough for FTE (full time equivalent) work for someone with that skill set, then that may justify part-time workers. Just remember that communication will be more of a challenge.
I agree to some point with @jmort253 that the full-time member is usually better, but:
Having a team consisting only of generalists able to do any work is a nice to have fiction. So there will be some parts of the project requiring particular person able to do it.
The part-time membership is usually driven by the scope of the work that needs to be done. For example the team may need a graphic designer only part time as there is not enough work or tasks for him to make it a full-time assignment in that team.
Mind also the difference between part-time member and part-time worker. Depending on the organization there may be some feature teams and service teams (like administrators, graphic designers). There will be no need to make administrator a full-time member in any feature team, however she will be a full-time worker providing "services" to different teams upon request.
Usually part-time membership in the team is driven by business/economical reasons and there is nothing wrong about it. It has the communication drawbacks that @jmort253 described, but if it suites your context best, then you should not be afraid of such solution.
Note that if you have this resource available for a specific period (either full or part-time), it might add additional complexity to your schedule. If the availability of this resource is fixed in time, a slight delay before that period can cause troubles. For instance if the deliverables that this resource needs to do his/her work is not yet ready.
On the other hand, if the resource is truly only part-time (eg only 50%, but for the whole duration of the project), I haven't experienced the issues as described by jmort and Marcin. Not in a high degree, that is. The main issue is to make sure that you get your 50%, as the other activities/projects might be eager to 'keep' your resource for just that 'little bit' more. It's also a pain to keep him focused; we all know the danger of multi-tasking.
Finally, you need to 'control' the time reporting more closely, since many people just book 50/50, even if the actual work done is more 40/60 or something.
Please forgive me but this question disturbs me.
I believe that after the last economic crisis the US and the other countries had experienced I think it is safe to say that saving on the little employee is always fun but generally not useful. I would elaborate but this is not the place and you only care for a simple answer anyway...
A guru... a super professional that costs a lot should be hired on a part time job if not on a freelance contract. Most of them even want this because they get bored on routine tasks.
For "normal" people
They like to have one supervisor - less overhead (no one likes to report to many supervisors; explaining to each why he has not finished his tasks)
The organization itself suffers a large amount of wasted setup time. It's hard to setup meetings, and it always take too long to recap and decision coming from individuals rather then a group is always more complicated.
- An Employee that feels unsecured even if he is well appreciated does not see himself as part of the organization. Today he is here, tomorrow there and the day after in other company (it's not about being loyal to a company - but to a project)
Yes I'm coming from the project management world and the part time job is crucial when a projects "needs" help or when an employee is not needed... In both assigning an employee to a part time job is serving both the organization and the employee best interests.
Yet, while being a great temporal management solution. It's a very bad long term solution.
Extra note: The cost of training a new employee instead of the old one usually cost more then the saving of a few hours(partial job). It can be the loss of time of the mentor, It can be the training time which bring no value and it can be the loss of bonus for early delivery(which is possible with extra human force available.)
Humans are not machines...The most effective companies lost half of their value due to social reasons: people that lost their job stopped buying. Try to see the bigger picture.
Because people don't usually have the opportunity to choose their team, I'll take the answer in a different direction. If you have a part time resources, remember that they are full time on the project when they are working on it. Try to negotiate the time as full days so they can be more productive and it will make your scheduling easier.