How frequent is the use of spoilers during descent?
Very frequent.

Modern airliners are certainly fairly slippery, but it's not so much that they're super-gliders that we can't wrestle down onto the ground.

It's more a function of busy airspace than a property of the aircraft. When you remove power from any aircraft, it's going to descend - the only question is the rate. I promise you that if you found yourself a long way from an airport in a powerless B737, you'd find that descent rate to be faster than you'd like!

The main reason for spoiler use isn't that a modern aircraft glides "too well" - but rather than the airspace is busier so aircraft are expected to respond a little quicker than in the past, which can make it more convenient to descend faster as a pilot. It also means that ATC will often ask pilots to maintain a higher-than-ideal airspeed on the descent: if travelling at 160 knots, you'll descend faster than you would at 180 or 200 knots, for example... but more often these days, you're asked to maintain a higher minimum airspeed for longer. That's often convenient for ATC as it gets you where you're going faster, and it's convenient for you too, because you get to go home faster: but it means you're travelling a lot further in the next minute, making it harder to hit the descent rates required, necessitating the use of spoilers in some circumstances.

Clarification: In this answer I've focused on the later stages of the flight: "Slower = descend faster" is not necessarily a universal constant - aircraft glide best at their "best glide speed" which is a function of drag, weight, and airspeed. This answer assumes that, as for most airliners, best glide speed is in the region of 200-250 knots, and that an airliner is unlikely to descend at/faster than their best glide speed in the approach phases where speeds are typically in the 150-200 knot range.

At lower altitudes when descending under ATC, the above ATC-centric discussion applies. At higher altitudes, it tends to revolve around efficiency and company policy: the longer you can stay at cruising speed/altitude, the more efficient your flight will be: therefore it saves the airline money to descend as late as possible, requiring a higher descent rate. Spoilers, of course, help with that.

TL;DR: Some of it stems from the aircraft design being more aerodynamic and therefore better at gliding, but it's more closely related to ATC being busier and therefore keeping aircraft higher/faster for longer, which requires higher descent rates and therefore the use of spoilers, along with the fact it's more efficient to stay high/fast in the cruise for as long as possible then descend later/faster