Reasons for single target
There is one reason: no storage space fragmentation.
No need to plan capacity in advance. As @Zypher points, NTFS is not a clustered filesystem, therefore you will have only one initiator communicate with the target.
You can configure the initiator host as a CIFS host for the other servers to share the storage. Since many database engines such as SQL Sever 2005 refuse to work on network attached storage volumes, your best candidate for the initiator role is the DB host. Exchange and File storage can use CIFS.
Reasons for multiple targets
- No dependencies between servers
If any single server goes down the rest will work as if nothing happened.
- No locking latencies or consistency issues
CIFS or NFS cannot provide latency as good as a block storage.
- Thin provisioning eliminates the fragmentation issue.
Many Disk Arrays nowadays have options such as thin provisioning that makes it easy to let several logical volumes grow into single storage area so that each volume can grow until there is free space left in the storage area without any interference on behalf of the Storage Administrator or host operating system.
Random files are probably not as critical as DB or Exchange for the business and tend to be more difficult to plan capacity for. It might be cheaper to have them on the capacity tiers while it is preferable to have database files on the performance tier unless the database is small and works from memory.
Even without thin provisioning logical volumes can be extended to accommodate the growth of data, but this would require interaction on behalf of the Storage admin and the admins of the servers.
If you can plan capacity in advance or the storage system has thin provisioning option, or storage tiering is important it is advisable to choose multiple targets. If neither of this applies choosing a single target sharing it with CIFS may be a viable option.