Clark Lindsey has some thoughts on the Russian space tug:

A tug might also make practical a single stage to orbit RLV. Since a first generation SSTO will most likely provide a very small payload capacity, it would help if it only had to reach a low orbit where it would transfer cargo/crew to a tug and also pick up cargo/crew to bring back from orbit. Even with small payloads, the simplicity of SSTO RLV operations might lead to reduced LEO delivery costs when combined with a tug.

Yes, this will almost certainly be necessary, in fact, if SSTO is to become feasible with anything resembling current technology. Any SSTO vehicle has very poor off-design performance. That is, if it’s sized for a low-altitude (or a low-inclination) orbit, the performance drop off for it to go higher in either altitude or inclination is very large. For example, one could have a vehicle capable of delivering ten thousand pounds to a hundred fifty miles altitude, that would have zero or negative payload to ISS or a Bigelow hotel). This is an intrinsic problem with SSTO, by the nature of the beast. Since there’s only one stage, the entire vehicle dry weight has to be taken to the final destination, so any additional delta V represents a big payload hit. A two-stage (or more) vehicle suffers much less, because the upper stage is much smaller, and is thus less sensitive to off-design cases.

OK, I hear you saying, aha! Then just make the space station mission the nominal design case. OK, now you just increased your development costs quite a bit, because it’s now a much larger vehicle. And once you’ve done that, you’ll still never take it to the station, because you’ll quickly figure out that it now has humungous payload capability to lower altitudes, that can be transferred with the tug. Regardless of vehicle size, you’ll get a lot more payload to the station if you use the tug (some of the extra payload is used to refuel the tug).

This also allows the station to live higher, which it would like to do to increase solar insolation, and decrease drag and monatomic oxygen degradation (the current ISS altitude is an expensive compromise between the desire to have the station higher, and the need to be able to get to it with the Shuttle). That in turn will result in reduced operating costs (reducing reboost and maintenance issues, and providing more power). I in fact proposed such an architecture back in 1982, in a paper I wrote while at Rockwell. NASA wasn’t interested.