What's the deal with idler turntables and do they have a place in modern HiFi?

I would state this differently: no single parameter can be optimized without taking the entire whole into consideration. Any architecture will always have strengths and weaknesses. Intelligently optimized, varying architectures converge but ultimately still carry their basic DNA (the architecture's key attributes - their strengths and weaknesses).

Cheers,
Thom @ Galibier Design 

What about this scenario:  Parameter is adding mass to (preferably the outside of) the platter.  Surely the only downside is the weight (and thereby friction) on the bearing.  If the bearing housing includes opposing magnets so that there is actually no weight on a thrustpad, then that parameter can be improved without any negative downside.  Or is there some other thing that will to some extent mitigate the inertia value of the extra mass?

I have a lenco modified nantais turntable that exceeds anything i have ever owned.

When An Idler Drive TT has undergone a 'Correct' amount of Servicing Procedures and as a further measure maybe had a Stock Platter Bearing exchanged for a different Platter Bearing, that is with good reports as a part and seen to be a Upgrade, if all is done correctly the Idler Drive TT is a very attractive TT for the Price to Performance ratio.

A GL 75 is proving today to be a very affordable Base Model TT with many affordable parts available to Support Servicing and original parts
have a reasonable supply of Exchange Parts that can be used.
A Web Search would show how to discover the wealth of information available to make informed decisions for a project.   

When an Idler Drive such as a GL 75 is used in the Serviced condition as above, it will sit comfortably as a comparison in the company of a Model from the usually referred to Idler Drive Brands.

As a comparison to other TT's with different Drive Systems, it would not be a Speedy Exercise to discover a TT with an Alternate Drive to show a Major Improvement in a Performance, especially near the same cost outlays.

If a Off Board Speed Controller is coupled with a Idler Drive,
the noticeable lift in the Performance will make the challenge to Improve on its performance, when compared to Alternate Drive TT's a even bigger search to find models to surpass it, with similar costs.

There are to the experienced ear, as well as my own,
that has experienced various TT Motor Drives methods as side by side comparisons,  noticeable differences in the Delivery of the performances from Idler Drives, Belt Drives and DD's.
The preferences expressed by individuals for the unique deliveries are choices to be made by the end user, and the Delivery that is determined to be the most satisfactory and is assessed as offering the most satisfaction for a Performance and Budget Allowance, will hopefully be the factors that help with a end choice.  
 
I personally know GL75 owners/enthusiasts and admire their work carried out on the GL TT's, through travelling to Social Events, I have met other GL enthusiasts and have been impressed with their works undertaken.
I also know and visit Two TT Owners, one with a SP10R and one with a SME that have adopted the GL75 as a Second TT in their Systems. 

In my limited worldly experience, it does seen a Idler Drive TT can become a desirable TT to experience and for certain types become Wed To. 

Andrei,

At least on a theoretical basis, some designers actually do not like high mass platters because they retain (store) vibrational energy imparted on the platter from the record itself which is vibrating because of the stylus tracking the groove.  Audio Note, for example, uses a light weight acrylic platter, but achieves the benefit of high inertial mass and high torque motors by using three large motors (belt drive).  I have no idea if this particular design is what gives their top end tables their particular sound, but, they do have the kind of drive and punchiness that I hear with good idler tables.