There is no doubt that PST is the best file format to make your data accessible and portable. However, since introducing a Cloud-based service (especially Gmail), I don't think there is any need to store data in a local drive when you can access cloud data anytime and anywhere. This is probably the primary reason Outlook users want to import PST files to Gmail mailboxes. However, there is literally no possible way to import multiple PST files to Gmail mailboxes with all freedom (exclude/include desired data, resume process, etc.). 

I use the Shoviv Gmail backup and restore tool for the same. It is like a blessing for Gmail users, as they can import PST to Gmail and export Gmail to PST in a single dashboard and cost. Along with this, it has several user-oriented features which I didn't found any other software, such as:

You can add multiple PST files of any size and process them into Gmail or vice versa in a single batch. Filter option to include/exclude required emails from the PST files. 

One thing that holds this tool in high regard is you can set a time limit to download an email, and if any email takes longer than that, the software will skip that. 

For other features evaluation and import PST files into Gmail with the first 50 items per folder, use the tool's free version. 

Read also – Export Gmail to PST