None of this is authoritative, please feel free to correct me anyone who knows more than I do...
1) Capturing a G/S from above is not illegal. It is highly unsatisfactory from a safety point of view, however, for various reasons
- You are in a low-energy situation, with low power.
- Your rate of descent is high and therefore, on capturing, needs to be reduced hard just as you are needing to pile power on to maintain the G/S
- You're heading towards the lumpy bits faster than was ever intended
- As you say, there is a possibility of capturing a side-lobe
2) Yes, it is happening more often.
3) Various factors to blame
- Excess speed (cf debates on a certain LCA elsewhere)
- Increasing load on controllers and airspace
- Less familiarity nowadays than hitherto among controllers with aircrafts' requirements
4) Most FD/AP systems require the aircraft to have locked on to the LZ a certain time (anything up to 30 secs depending on type) before they can capture the G/S. If an aircraft is vectored onto the LZ too close in, they have less chance of establishing in time. The G/S comes and goes, and you then have to chase it down. Altenatively, at too high a speed or too tight an intercept angle, you go straight through the LZ, have to turn and reintercept from the other side. But you get the picture.
So, all things considered, more pressure, more crowded skies, and aircraft not helping themselves by maintaining too much speed.
For safety's sake, what should really be done if above the G/S and too close in is ask for vectors back onto the LZ slightly further out. But TMA's are busy nowadays and companies won't like the added delay.